Try it when you want to control who is speaking and when.
Try it when you want to co-ordinate hundreds of participants and still want to track who has a question so you can hand the virtual mic / airtime to them.
Try it when you want breakout groups and to determine who is in which group, and after a set time for the groups to return to the main space.
What is good enough for 2 people facing each other, and appears to work perfectly well for a group of 5 or 6... doesn't quite scale to a company all-hands, or giving a lecture or seminar.
Tools fit a scale, and Zoom is excessive for the small and simple use-case but excels at the large and complex.
I don't quite understand why Hangouts and Skype aren't more robust than they are. I'm sure this is indeed a hard problem, but the utility of getting it right is obvious, and these are massive companies.