It should also be seen in context. Certain celestial bodies (moon and the sun) have a massive impact on your life on earth. It is easy to see why people would interpolate this to other celestial bodies.
I mean, yes, that's true.
But IMO it's not really at random. I don't think anyone would dump someone because that person is a Scorpio (unless they're absolute nuts). It's more like a conversation starter to let people open up and talk about their personality traits.
You bring up jokingly "ugh my ex was also a Leo... Leo are so dramatic yada yada" and you go "yeah I get it but I'm actually chill, maybe it's my Sagittarius ascendent" or something (I'm talking out of my ass here, dunno if those examples make sense... but then again I don't believe in it, so...).
It sure beats talking about the weather in the small talk scale.
Yep. I'd be surprised if there's any!
Still a fun icebreaker.
I guess it is useful to know when someone can't entertain hypotheticals, make light conversation, lacks sensitivity about other's world view, and is allergic to beliefs which are not promoted by large organizations.