Qubes Manager GUI & UX
It's standard in most GUI program to have a button which resets all options to their default settings, where "default" means roughly "how things would look on a fresh install."
What exactly do you have in mind? "Reset VM settings"? Or something more
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?
Qubes Manager --> System --> Qubes Global Settings
In this window, the user can change things like: minimal VM memory, dom0 memory boost, whether to keep DispVMs in memory, whether to check for dom0 updates, and whether to check for VM updates. A user might change some of those settings (being new to Qubes, not realizing that her custom choice is a bad idea), then later realize that she wants to revert to the defaults. But maybe she doesn't remember what the defaults are. It would be nice to have a button which sets everything to the defaults.