Many Android devices offer some customizations, though the exact parameters vary. Samsung, for example, offers several built-in options, such as color and font selections for certain UI elements. These settings can be further tweaked using tools like Good Lock (found in the Galaxy Store and only available for some Samsung models) and its related apps. Other manufacturers might not give you as many choices, but with a rooted device, you can install theming apps and other customization tools to tweak almost everything in the Android UI.
Another popular reason to root is removing bloatware — built-in apps that you can only disable at best (and sometimes not even that). A rooted phone will let you remove basically any app you want. Furthermore, you can install any app you like. Even sideloaded apps have some limitations, especially on the newest versions of Android, but on a rooted device, you can install anything you want.
You might even find custom Android ROMs for your specific device that make it faster or more powerful. Sometimes both. You can also get full OS backup (also called cloning) on a rooted device, something Android itself still doesn’t offer — some settings and files simply aren’t transferable due to OS or app restrictions.