Even if that's just a seam, so to speak, it doesn't mean it can't evolve, I guess.

So the question is: why do we have a part of the body that seems to serve no purpose besides getting infected? And if it is not, in fact, useless, what purpose does it serve? Logically it should improve survivability more than the occasional infection decreases it, otherwise it should have disappeared, or at least, be different than it is now.

Unless appendicitis is a relatively modern disease and didn't affect survivability significantly and for long enough for evolution to do its work, which is a common hypothesis.