And though we probably shouldn’t expect to see extra-planetary visitors dropping by Central City P.D. on a weekly basis or anything, having the DEO up and running would create an easy way to make that happen, not to mention getting more big government influence into Central City.
Up until this point in their respective stories, aliens hadn’t been introduced into the narratives of Arrow, The Flash or Legends of Tomorrow.
(We assume that the rest of the planet was also freaking out about it, but not a lot of outsider response was visible.)
Barry clearly had knowledge of them, thanks to his trip to National City last season, but getting both Supergirl and the Dominators was quite a lot for the rest of the heroes to handle.