Why do people install Cyanogen Mod in Nexus smartphone when their phone is already powerful with Stock Android and Android updates?

I don't remember the last time that I used an Android without the intention of rooting it and putting Cyanogen Mod on there. Seriously, once you figure out how to do it (which, again, can be a frustrating process at first), if you're anything like me you won't be going back. Cyanogen Mods allow you to do so much with your phones: Giving your phone an extensive makeover with complete skins. Performance boosts and even being able to upgrade software versions before it's officially available for your phone (if ever). In short, Cyanogen Mods can make your phone seem like an entirely new device, on the inside at least, without actually having to go out and purchase a new device every time a new one comes out. It can satisfy the serial phone switcher's unsatiated hunger for change in mere minutes once you get comfortable with the process.

Sometimes after a period, your phone will stop receiving updates, and it's best to just flash a ROM in order to gain the latest features. Like for instance, the Nexus S and the Galaxy Nexus will not receive the update for KitKat. So in order to get KitKat on those devices, you have to flash Cyanogenmod. And CyanogenMod has a ton of more features than the original ROM.

Yes! the stock ROM is no doubt powerful. But one of the best power of Android and Nexus device is full customization! The word "full" literally means everything, from different ROMs to different kernels and much more. Installing a new ROM like CyanogenMod or ParanoidAndroid is a part of that process. Cyanogen looks almost same like AOSP but it has some good modification over AOSP : 1. My favorite one: DSP Manager :: It helps to manage how your mobile sounds when you are hearing musics or consuming digital media. There is no dsp manager associated with the stock build. You have to rely on some other third party apps to this. But with CM, its too easy. 2. Inbuilt file manager. 3. Full customization of themes and various look and feel. 4. Some optimization in kernel targeted for specific device. There are some other advantages also, but I can remember this things now.