Ideally yes, but for whatever reason, many games posted here don't have any styling at all. Whether it's because developers don't know they can style their pages, or their games just don't have much in the way of graphics (like interactive fiction or game jam projects), I can't say. Much like other website themes, convenience is the key. Having a pre-built theme that can be easily customized to include your logo or other art can potentially save hours of work, hours that you can spend building your game.
The pack includes not just background graphics but also animated horizontal rulers and HTML style guides to get the best colors for all of itch.io's options. We've designed these to be easily customized by the end user as well, and they are distributed under a Copyleft license so users can distribute, and even sell, their own themes if they wish.
The goal for these is to be a time-saver. Whether users decide they're best used as temporary backgrounds while their game is in Early Access, or want to use them as their page's permanent look, is up to them. So I absolutely agree but developers should promote their own art in their themes, but if they don't want to spend time building a theme, see something here they like, or feel like 10 minutes of customizing what we've built will work for them, then we are here to help.