It used to be the case that device nodes for serial ports and lock files for those were owned by that user/group. Historically uucp (uucico - uucp call in call out) was the only sensible thing to use the modem for anyway...

Personally, I never used uucp over serial links, but for several years fetched my emails via uucp-in-tcp. It was a homegrown system with sendmail, later qmail, later exim and it worked very reliably.