Speaker cable connectors? What exactly do they do?

Actually the best connector is no connector. A direct connection to the crossover wires inside the box is best. Some people actually do this and there is a noticeable difference in clarity. Other than that, maximizing the contact area of the connection is the 2nd best thing to do. Most feel that spades accomplish this, but a TIGHT fitting banana is probably as good.

I agree with Jig that no connector is best although wiring directly to crossover seems a tad difficult. I prefer to use bare wire connected to speaker terminals but use bananas to amp to facilitate changes in cable.
I would bet most Agon members don't keep the same speaker cables long enough for corrosion to build up. I know I don't.

Bare copper will oxidize over time. Oxidation is a real problem. It will rob
you of signal over the entire spectrum. You can use bare wire and simply
snip off the ends every once in awhile, trying to snip them before oxidation
becomes a problem. Connectors, if they're applied correctly and if they are
plated with a material that won't oxidize -- like gold -- can provide a better
seal and help fight oxidation. Banana plugs make it much easier to connect
and disconnect your speaker cables. Spades are a tad less convenient. Some
people say they can hear the difference between spades and bananas,
between connectors and no connectors. I usually take claims of super human
hearing with a grain of salt, but that's just me. If you think you can hear the
difference, take a blindfold test and see if you really can or if you're just
kidding yourself. Then, proceed based on your results.

But how does adding connectors to the end of the cables keep the copper from oxidizing?