All kinds of repairs, hyperspace calculations, course-plotting ... A lot of starfighters use astromechs as co-pilots. [Which is the real reason she wanted one, though she's not going to tell Kitty that considering the woman's reaction to their last conversation on the topic of Rey leaving.]

I can't be sure until I boot her up again, but I suspect she's quite a warrior. She's been with the Resistance for over a decade. [With a droid so old, actual replacement parts were extremely difficult to come by, which is why the mechanic had reluctantly decided to scrap her before Rey came along.]