{Why is there a base and jumpgate on Io.}

Yes, this is the primary jumpgate for Earth. It's positioned at Io (most of
the base is actually in orbit outside Io, some of it on the surface, as we
showed in "Chrysalis") to keep Earth a bit safer by removing it some
distance, and because putting it near the huge energy field of Jupiter makes
it more difficult for weapons to target it from a distance. It's mainly a
tactical decision.

jms