This is often done for consistency. Especially (although I have to admit that does not apply to TOS) when also heavy makeup or prosthetics are applied.
In Voyager, every main cast member is wigged to some extent, even if just false sideburns. Except apparently the Doctor who grew his own pointy sideburns and was clearly not bewigged on top.
Robert Picardo talked about this on I think a podcast I listened to several years ago, so sadly I can’t link to the source as I don’t recall where it was.
