It would make it that much harder to justify many religious beliefs, which I predict would be a positive outcome. It's harder to be arrogant and self-righteous when you find out you're not an only child. Sure, some people would find a way to stay stuck in their ways, but not everyone.

However, the much more important thing for me is that we'd know we aren't necessarily going to run into some Great Filter [0]. I'd fret a little less about dangerous new technologies if I knew at least one case where a civilization survived.

Lastly, maybe we'd learn something from the signal itself! A new communications protocol, new music, new technologies, insight into how languages work - who knows? Finding out what we have in common with the beings who sent the signal and what we don't would teach us a great deal about what is arbitrary on our world and what's fundamental. It's like growing up in a society with extremely rigid gender roles and then traveling to a country where that's not the case - you get that "Oh I didn't realize women could even have jobs" moment, but potentially with problems that aren't self-imposed.

0. https://en.wikipedia.org/wiki/Great_Filter