HTTP traffic would be unencrypted, so everyone (esp. on Wifi) could record passwords etc. flying around. With HTTPS, you at least need to MITM the connection to do that. If you establish trust in some other way (cert ID printed on the device?), the connection is secure.
Not just intercept: Using HTTPS prevents messing with the connection in-flight. So an attacker won't be able to inject their own payload into that web page you just requested.
That's a separate kind of attack, not intrinsic to the protocol. If the keypair got leaked or a CA misbehaves and issues multiple certificates that can be used for a host, then yes, it can happen.
