Some do. It depends on the type of certificate. Thankfully now we have LetsEncrypt so that there is a free alternative to the big CAs.
To answer your initial question - yes it is necessary. Without HTTPS or encryption in general, anybody who can intercept your connection can see everything you're doing.
A real world example of this is let's say you're connected to a WiFi network that has no password and are browsing a plain HTTP site. Open wifi networks are unencrypted, as is HTTP.
I can sit across the road in a vehicle, unseen, on a laptop and sniff the traffic to view what you're doing. If you log into your bank, I now have your credentials and can do what I like, and you don't even know.
This is why we need encryption. It is an (almost) guarantee that your traffic is only viewable to yourself and the other end of whatever you're connecting to and not anyone in the middle.
Edit: for Anyone downvoting OP remember this is nostupidquestions. Take the time to educate if you know better but don't downvote "stupid" questions lol.