I always figured that starfleet ships would ship starfleet cargo around. It would seem unusual to ship around civilian people/things unless in emergencies.
Yep, Starfleet ships in general also double as cargo/supply ships. This was done alot in TOS and in TNG we still see the E-D making deliveries.
Civilian/commercial freighters, cargo ships, cruise liners etc exist but they're not the focus of most Trek series and thus we don't see them as often, IMHO.