Make sure to always create a backup of your Android device before proceeding.
Installing a custom ROM requires you to unlock the bootloader first, and have a custom recovery like TWRP ( website) installed. While there are many custom ROMs out there, not all of them are stable enough to be used as your daily driver.
They also reduce bloatware that comes as a part of many manufacturer skins. A custom ROM, on the other hand, helps keep your device alive and updated with the latest version of Android. Another reason why people seek custom ROMs is because of the extra features they offer. The main reason why people prefer custom ROMs is that almost every company drops support for their devices after 2-3 years of their launch.