It may of course end up being some sort of dumb error, or the result of Earth-originated pollution.
If there is life on Mars we need to find out right away the extent to which it has affinity to life on earth. Either result would be exciting. If life on the two widely-separated planets has affinity, this would support Chandra Wickramasinghe's panspermia hypothesis that Earth was seeded from space. It would at least prove that such seeding was possible (in that one of the two planets must have seeded the other, at a minimum).
On the other hand, if the life on Mars is of a completely new kind, this would imply that the chances of life arising in any congenial planetary niche are high.
Either possibility therefore is very exciting for those of us who want to believe in life out there.
And finally, all Earth missions to Mars must from now on operate under sterile conditions.