What are some jobs in the army where you don't just watch a civilian do the same thing and you actually have a purpose garrison?

All Combat Arms and Combat support. There are actually a litany of MOS's in which civilians are not qualified nor utilized in that capacity. Speak to your in service recruiter/retention NCO.

Preventive Medicine

Unit supply, CBRN, Commo (25U)
Never seen a civilian supply sergeant in a unit