Most Android devices offer customization, although the exact parameters vary. Samsung, for example, offers several built-in options, such as colors and fonts for some UI elements. These settings can also be changed using tools such as Good Lock (available in the Galaxy Store and only available on certain Samsung models) and its associated apps. Other manufacturers may not give you many options, but with a rooted device, you can install custom apps and other customization tools to change almost anything in the Android UI.
Another common reason for uninstalling is to remove bloatware – built-in software that you can easily disable (and sometimes not even). A rooted phone will allow you to remove literally any program you want. Also, you can install any program you want. Although the limited apps are limited, especially on newer versions of Android, but on a rooted device, you can install whatever you want.
You can also get custom Android ROMs for your device that make it faster or more powerful. Sometimes both. You can also get a full backup of the OS (also called cloning) on rooted devices, something Android still doesn’t offer – some settings and files can’t be changed due to OS or software restrictions.