Android is open source, so developers can take its code, add features, and build their own operating system images for Android phones and tablets. Many Android geeks install such custom ROMs — but why?

“ROM” stands for “read-only memory.” A custom ROM replaces your device’s Android operating system — normally stored in read-only memory — with a new version of the Android operating system. Custom ROMs are different from acquiring root access.

Get The Latest Version of Android

This is by far the most popular reason to install a custom ROM. Many manufacturers never update their older Android phones and tablets or updates may take months to reach phones thanks to carrier and manufacturer delays. If you have an older device that isn’t receiving updates anymore and you want to run the latest version of Android, a custom ROM is just the ticket. CyanogenMod is the most popular ROM for this purpose — it has its own tweaks, but the base system is similar to the stock version of Android created by Google. Thanks to CyanogenMod and other custom ROMs, many older devices that will never be officially updated can run the latest version of Android.

If your device is still receiving timely updates — especially if it’s a Nexus device that Google is updating regularly — custom ROMs won’t be anywhere near as compelling.

RELATED: Why Your Android Phone Isn’t Getting Operating System Updates and What You Can Do About It

Replace Manufacturer Skin With a Stock Version of Android

RELATED: Why Android Geeks Buy Nexus Devices

Manufacturers like Samsung and HTC “skin” their versions of Android, replacing the clean look Google created with their own look that’s often more cluttered and less cohesive. Many people don’t like this but still want to use a flagship phone like the Samsung Galaxy S4 or HTC One.

You can’t just switch from the manufacturer’s skin to the stock Android look — sure, you can replace the launcher without installing a custom ROM and even without rooting, but there’s no erasing all the questionable customizations the manufacturer has made to the operating system. To get the stock Android look and replace all the manufacturer’s customizations with the clean Android system, you’ll need to install a custom ROM.

If you don’t mind your device’s skin or you’re using a Nexus device that already comes with a stock Android system, there’s no reason to do this.

Eliminate Bloatware

RELATED: How Carriers and Manufacturers Make Your Android Phone’s Software Worse

When you purchase a phone from a carrier, it often comes packed with bloatware. NASCAR apps, TV apps, a Contacts app that stores your contacts on your carrier’s servers instead of on your phone — these apps can clutter your system and waste disk space. Manufacturers even add their own software before the carrier gets to it, so you have two companies each adding their own bloatware to your phone before it gets to you.

If you want to actually erase these apps from your disk, the best way to do so is to install a custom ROM. You can disable the apps without rooting, but this won’t free up the disk space they consume.

Add Additional Features and System Tweaks

Custom ROMs offer features not found in stock Android and many tweaking options you can’t get elsewhere. For example, a custom ROM may allow you to:

Install skins to customize how your entire Android operating system looks. Customize the quick settings menu Android includes to add your own most-used settings shortcuts. Run apps in tablet mode on a phone, using a more full-featured tablet interface for certain apps. Easily overclock your device to make it run faster or underclock it to make it run slower while squeezing out more battery life. Disable the volume warning that Android constantly shows when you increase the system volume while headphones are plugged in. Hide the bottom navigation bar (on-screen buttons) to get more screen real estate. Easily enable root access by toggling a system setting.

Custom ROMs offer many other features — this is only a snapshot of what you can do with such low-level access.

RELATED: Forget Flashing ROMs: Use the Xposed Framework to Tweak Your Android

Some of these tweaks may be possible on a typical Android device with a solution like the Xposed Framework, which allows custom ROM-like tweaks with only root access. However, custom ROMs are further along in development and include these features in a single package.

Configure App Permissions

Custom ROMs often include a way to manage Android app permissions, so you can prevent Facebook from tracking your GPS location and play Android games without giving them your phone number and other identity information. This feature showed up in Android 4.3 as a hidden settings panel, so we can only hope that it will appear in an official version of Android soon.

Reasons Not to Install A Custom Android ROM

Custom ROMs aren’t perfect and they can have downsides — depending on the ROM, your device, and how well the ROM supports it. You may run into:

Battery Life Problems: The custom ROM may not be as optimized for your device and may drain battery faster than the device’s official ROM. Hardware Issues: Custom ROMs may not properly support every bit of hardware in your phone, so you may run into bugs, non-functioning hardware, or just other issues. For example, the device’s camera may not take pictures quite as well as it did on its official ROM. Bugs: The custom ROM hasn’t been tested by your manufacturer and carrier, so you may run into other bugs specific to your device and ROM. You could also experience system instability, with apps force-closing and the phone randomly restarting itself.

Custom ROMs are also more work than just purchasing a device and having it be officially supported and updated by the company you purchased it from. That’s why many Android geeks buy Nexus devices, which receive timely updates directly from Google. CyanogenMod is trying to change this by offering an easier installation process via a CyanogenMod app in Google Play.

RELATED: How to Flash a New ROM to Your Android Phone

If you’re looking for a custom ROM and don’t know where to start, check CyanogenMod’s website and see if it supports your device. You can also check the XDA Developers forum for your Android device and find custom ROMs developed specially for your device, which can be helpful if you have a less common device. Be sure to find a ROM that appears to be stable and well-supported if you go this route.

Image Credit: Jon Fingas on Flickr, Johan Larsson on Flickr