If made correctly (which is hilariously easy), it’s a clean install and uninstall process, support some level of potential conflict regarding files that are shared with other packages/commands, support dependencies out of the box, and with minimal work can be made easy to update for the user (even automatically updates, depending on the user’s choices) by having an (again, very easy to setup for a dev) repository. With the added value of authenticity checks before updating.
All this in a standardized way that requires no tinkering, compatibility stuff, etc, because all these checks are built-in.
Note that some of this probably applies to other system package management solutions, it’s not exclusive to .deb.
When I switched to Ubuntu, they just had more up to date packages, and with two releases a year (sort of), stayed up to date with other software, which is a good thing for a system I actually use. From then on, I just stayed on it, because I don’t reinstall my OS until something’s broken. I’ve been moving the same one for a decade now.
If I had to install a new desktop system I’ll probably go with mint, for the same reason : more frequent software update.
Note that this is all for desktop (and some specialized systems). Servers are all running debian, because stability is preferable and frequent software change is not what I want in these environments.