It’s just peace of mind that the button exists.
I was doing tons and tons of tricky edits on a site this morning and we’re upgrading plugins, need to update WP core, etc. Knowing that the site was hosted at WP Engine and that I had a “Restore Backup” just a simple click away was a peace of mind that I (not to mention my client) found extremely comforting.
Yes, of course, you can do backups in many other ways, but it’s far from one click restore. It’s also daily (automatically), right there in your WP Engine dashboard and you don’t have to worry about taking up too much of your account disk space.