They also assist in setting the brake pad clearance from the rims - I grip them tight onto the rim, pull the brake cable tight, tighten the cable bolt, and release them. Usually there is enough slack in the system that the pads will just clear the rim without requiring further adjustment.
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1989 Schwinn Paramount OS
1980 Mclean/Silk Hope Sport Touring
1983 Bianchi pista
1976 Fuji Feather track
1979 raleigh track
"I've consulted my sources and I'm pretty sure your derailleur does not exist"