1. the photo lab misses a few frames. I don't know why, but maybe it has something to do with the scanning mask they use.
2. non standard sized negatives from cameras such as Sprocketrocket, DianaF+ 35mm back square w/ sprockets and panorama w/sprockets and Diana mini tipster (get around 50 shots on a reg 36 exp roll).
3. save money
4. save time (with some labs you get the negatives after a few hours but if you want the scans, I'll take a week)
5. more control (adjust exposure, color levels, contrast etc.)
Initially, I was pining for Lomography's smartphone film scanner since it can scan 'everything' (yup, it can even scan 120mm) but the cost is prohibitive (about $80 if I get it from a local authorized seller), so I ended up buying a non-lomography one and I'm glad I did. Then I fell in love with sprockets, unfortunately my trusty non-branded scanner couldn't handle that.