I use them when I am buying only to catalog. It is easier, and TIY stays open just as late as ReTail. When I am working to make bells, though, I sell at ReTail.
The Nooklings shops are more meant for buying than selling, which is why they have so many upgrades if you buy. Once you get GracieGrace and T&T Emporium, you'll start going there a lot more.
The first few iterations of their shop opened earlier than Re-Tail, so I guess that's sort of an advantage if you really needed to sell something in the morning. Other than that, I don't think there's any advantage.
The nook brothers sell your stuff to retail, which is why they charge less, you see this when you 1st enter retail. My main reason for selling to them is it is quicker to get to when I am buying everything from able sisters and kicks, rather than running back to town to retail, and the money from that stuff is hardly anything no matter what.
As far as selling goes, they offer prices 25% less than what you can get from Re-Tail. Possibly for the sake of convience if you don't want to trek from Nooklings to Re-Tail.