Roms are usually used for 3 major reasons
- prolong a phones useful life
- a way to optimize the phone by removing bloatware in /system
- to better suit a persons needs, i.e. security (quicker security patches), privacy (for the degooglers) customizability, pixel exclusive features, etc.
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 .