But afaik almost everybody uses it as autocomplete on steroids.
So the reasoning behind using it is the exact same as using autocomplete in I.e. Intellij.
It's just slightly more powerful, since it will usually autocomplete the whole line (occasionally a bit more), instead of just the current keyword.
I do find it's much better in some languages though. And probably especially with languages you're new in. It doesn't do much for me in PHP, but does a lot in JavaScript.
And it's scary how good it is for "autocomplete" sometimes. I've found it much more useful on our angular webside frontend rather than the c# backend.. but man on the angular/javascript side it really saves time.
I can't think of any good examples off the top of my head, but every few days I literally am wondering to myself "how did I now I wanted to do exactly that!"