What does the retro preset do?

As above, what does the retro preset do that makes it special compared to the other presets?

Nothing. The preset dropdown just fills in the viewport size and orientations fields, so you can see it just sets a resolution of 320x180. For retro games you can also check 'Optimize for pixel art' to set various project settings suitable for retro games, and if you click the 'Help' link on that dialog the manual explains what it does.

That's what I thought.

If I check the Optimise for Pixel Art check box then I assume that expanding from a small resolution to a l60'' HD display would be fine, the boxes will be bigger but will remain crisp.