The biggest advantage to me is not having to deal with the steaming pile of rubbish that is the KDE Store and not having to install Discover, which is also rubbish.
I already wrote about that:
I’m comfortable using the AUR, much more than I am comfortable bypassing package management when installing stuff. I don’t even have Discover installed. Now, there is less choice on the AUR, but the good ones are there. And the whole thing is taylored to Arch users. If it is a popular Plasmoid, chances are good it will be maintained, and if it is outdated, it will be flagged out of date, and you’ll know it’s outdated. On the KDE Store, you can still find Plasmoids and themes from the Medieval period, not marked as out of date.