Can someone please enlighten me?

I am having trouble understanding the myriad communications/chat apps out there that don't seem to do much more than what we older folk used to do on USENET.

Then there's all the clones of more modern platforms. Things like Whatsup.

It just seems to me there are, as a friend of mine used to say, "too many useless duplicates". Meaning, most of it is a huge waste of time.

So, what am I missing about Slack specifically? What unique differentiation makes this worthy of anyone investing their nest egg on their stock?

I honestly want to know. BTW, I have used Slack. I think it's horrible. I don't see much improvement over, say, a phpBB site (which can be deployed privately inside of a network or on the internet).