A couple reasons I can think of: - The less logic you do on the client means more data required to be sent over the wire. - Connections are getting faster but we aren't at the point where they're negligible.

For example, if I need to do complex form validation in real time I could send the form value to the backend, have it validate, and receive a response which introduces a lot potential of failure points. Alternatively I could write some logic on the frontend to validate, and this problem expands as the amount of fields in your form increases.