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.