HTTPS provides some very important security properties that HTTP does not.
It provides confidentiality, integrity and authentication.
- Confidentiality: What you're loading cannot be snooped on by others. Eve is on the same networking as you, and can listen into your traffic - but if it's HTTPS they cannot read it. If it was plain HTTP they could see what you were looking at and copy your session cookies to potentially log in to sites as you.
- Integrity, Authentication: You know that if a document you recieve is valid it is unmodified. Mallory cannot set up a man in the middle attack and edit the documents in transit, if it was HTTP they could do this with arp poisoning/cain and abel - and change all the images on the pages you are looking at to rick astley.
Implementing confidentiality requires a cipher and a system for keys, initialization vectors and such. It's rather involved.