UX. It's much faster to have JS return an instant error because you forgot an @ in your email than to have to do a roundtrip to the server and watch the page flicker (reload), especially on a crappy connection.
Not so big a deal on one field, but if you have 5+ errors in a long form, it can add up.
You of course still have to validate server-side to stop ancient browsers or people trying to hack your site from submitting bad data.
Browsers which support HTML5 form validation are "suppose," to validate the input prior to submitting the forms. I think this trended a bit ago here on HN about email validation...even if you do a <input type="email">, a user can still submit an email in these forms (blah@you , 123@12.zz , 12345@localhost ,...get the idea).
Another quirk about form validation in HTML5 is the forms won't get validated UNTIL the user submits the form and then the browser may highlight the error in consecutive order (1 at a time o_O), imagine your poor UX for a user that has multiple errors.
Except that the form is already validated client side by the input elements, that's the point. It doesn't make any sense to validate it twice client side.
Not so big a deal on one field, but if you have 5+ errors in a long form, it can add up.
You of course still have to validate server-side to stop ancient browsers or people trying to hack your site from submitting bad data.