Roms are usually used for 3 major reasons
Personally, i trust operating systems that are free and open sourced (code that i can see, build, and audit myself) over code thats not being provided to the public. This has a few drawbacks but also quite a bit of bonuses.
At the end of the day, its truly up to the individual if they want to flash a custom rom, and they can do it for a multitude of reasons .
