What is this condenser used for?

Also that cap goes on the coil POSITIVE, not the distributor wire, and it actually does more than just supress radio. It gives the spark pulse which "tries" to showup on the + side of the coil a low impedance path back to the engine block. This "may help" to keep spark power and voltage "up."

DO NOT use these and REMOVE them if you are using an MSD or other capacitive discharge.

Also Halifax I disagree with your opinion on them. That is, they are old. Most these caps are leaky. It is very likely that most of them should just be replaced, but the problem is, that most you buy nowadays are low quality. A plastic dipped cap (like an "orange drop") would be a much more reliable replacement, but now we are treading water because I really LOLOL don't know a good source for RELIABLE and QUALITY replacements.