I needed something at the front to keep my passport, tickets, small camera (Ricoh GRIII), credit card, etc. in. I’m always having a bit of a faff for that stuff. Hopefully this should help.
Why not a tank bag? Did try to get one (a nice Givi) but had no end of trouble trying to find the right mount for the tank, gave up in the end and Claire now uses it.
Advantage for this bag is the fuel cap is left available at all times.