HSTS preloading offers the highest possible level of security, as the user's browser is enforcing the use of HTTPS. Merely serving via HTTPS is only optional security, as any man-in-the-middle attacker can strip that encryption (see sslstrip, released six years ago). For more information see my blog post from last year: https://security.googleblog.com/2017/09/broadening-hsts-to-s...
Preloading the entire TLD rather than individual domains has a number of benefits, including the fact that (a) it's effective now rather than several months from now for a newly created domain, (b) you don't individually have to configure anything, and (c) it keeps the size of the list down (which is important since the list is built into web browsers).
And in addition to all that, you're right, it is a play to move more sites to HTTPS, which is better for the safety and security of the web overall. Chrome is soon going to display "Insecure" for every single http site, which is another nudge to help move the web towards a secure future.