There's a pretty good split going on here. It's funny. I think I got my first external preamp because I wanted to record using a condenser mic on a recorder (can't remember the model) that didn't have phantom power. Since I'd been reading about external pres anyway, I decided to just snag one of those (an M-Audio DMP-3) instead of just getting a phantom power supply.
Back then (13 years ago maybe?), which was before I had set up a computer-based system and was still recording with standalone digital or analog machines (I still do the latter as well), I got the notion that even a modest external pre will sound better than the interface's built-in ones (not the super expensive ones). Even though I now know this isn't true, I've just gotten into the habit of always using an external pre.
I remember the first time I ever noticed a difference in the sound of a preamp was when I used to record to ADATs in the 90s. I was using a Mackie 1604 mixer and switched to a Soundcraft Spirit Studio. There was a noticeable difference in the sound, with the latter being much darker but having more character. I think that's when I first understood what people meant by saying that Mackies can sound "sterile." And I think I transferred that notion to interface pres (I'm not sure why) and so I'm always biased into thinking that I'm just getting a clean-yet-sterile sound with an interface pre, which doesn't appeal to me (generally). Again, I know this isn't fact-based really; it's just a feeling.
Anyway, with that said, I usually use a Seventh Circle Audio N72 pre as my first choice on most stuff. Otherwise, I might try my (significantly) modified Presonus MP20 (which I really like), a dbx 386 (which can add a little grit), or a vintage Shure M67 when I want a dirtier, earthier sound. I also have a modified Art Tube MP that doesn't a whole lot of use. Until recently, I had a Steinberg UR22 interface, but I just got (last week) a Tascam US-1800 --- haven't recorded anything on it yet.