The screw on the 5th string holds the whole silly thing together. If it's a friction peg, you have no gear ratio and keeping that little screw tighter will limit string slipage. The geared 5th string peg makes it easier to tune that string---sort of like, as you say, a fine-tuner. --- Right.

I always felt it was easier to tuna fish than the 5th string.
The 5th sting came to be when a fellow killed a possum to make stings from the guts, and that animal's innards were only long enough for four-and-a-half strings ----- so that one string only went half way up the neck.

I always used fiddle fine-tuners on my banjo strings----at least on the treble strings. Use the kind that ride the strings themselves---grab the string at two places, and then depress it in between when you turn a small screw.

Art