I'm talking here of the future humans (or whatever their evolved state made them) and why they created the wormhole to save this film's particular failed branch of humanity?
I don't think I ever read an explanation back in the day so I could never really get onboard with this film.
The 'answer' to your five year old question is, as always, that time travel is fundamentally paradoxical - if one could alter the past, one would inevitably alter the future and quite likely obliterate the existence/agency of the individual doing the fiddling. Or, on the fringe, create an alternative universe disjoint from the original, leaving the original 'problem' abandoned.
We love time-travel because its such a cool fantasy, but it is just that - a pure fantasy. At least in this universe.
And so, any film treating time travel will be, essentially, bollocks. Sometimes, though, good bollocks. Not, imo, in this case though - a pretentious mess. I love the SF I love, but end up hating most of it. Usually because the science is fictional.