If you want to use your phone for a few years it’s very nececary in my experience, they tend to get slow quick and with LineageOS (previously Cyanogenmod) I can still use a Galaxy s5, well I could if half of the hardware wasn’t broken but that’s beside the point. I think ROMs are definitely neccecary but mine isn’t rooted rn and it’s probably debatable if you realky need that, I miss a proper adblocker so I should have rooted mine and probably will sooner or later considering I got Lineage on it already.