On Bally games, they are required for any fast-react solenoids (that's slings and pops) to ensure that you get the score for the fast react solenoid. (Without them, sometimes you will only get the solenoid reaction and not the score as they are 2 different routines in the code). The slings and pops could seem to react faster with the caps because it would always activate on the first 2 reads of the switch instead of possibly the second and third (which is a normal switch response).

They should also be on any standups where there is a possibility of a very fast hit, like the standups, or sometimes rollovers. Those switches are not read by the fast react routine so there's no 'faster' reaction time; in this case, the switch pulse is longer, lengthening short switch hits slightly to ensure they aren't missed.

It won't hurt anything but be aware that sometimes stray EMF from holding a flipper up can be picked up by poorly made caps causing a phantom firing of the sling or pop when you let the flipper down. You won't get the score from the pop/sling but it can fire seemingly randomly.