There are a number of reasons:
– Security (in my opinion, the main reason): It allows for the data passing between the user and the server to be encrypted. This means that anyone else on the same network as the user will still be able to read the packets being sent, but they’ll look like gibberish unless you have the private key (on the server). So if sensitive info is sent, like a form submission with password or credit card info, no one can steal it.
– Trust: Some browsers (Chrome, maybe FF) started showing UI changes that tell people whether a website is secure (uses encryption) or not
– User experience: Some browsers (Chrome) blocks access to a website that dont have encryption set up and warn users that continuing could be bad.
– SEO: Some search engines (Google) penalize websites that dont use encryption