It supplies some voltage to the A coil at all times, so that the contacts on the disc (on the ones reel) don't have to take the coil all the way from 0 to 25 volts. I think this is so the contact points on the disc and/or the wipers are not subject to as much arcing. When the ones-unit wiper is not completing the circuit, the A coil is not pulled in.

Your excerpt of the schematic shows that one of the things the A relay does is switch the score values of the kicking rubbers back and forth. You should find another switch for relay A over on the lighting portion of the schematic where the "10 points when lit" playfield lights switch back and forth. Every 1-point score changes which of the kicking rubbers is worth 10, with the other being worth 1. There may be other alternating values elsewhere.
.................David Marston