Promoting Garrison units to Field Units costs Shmuckers, such as when Queen Bea emptied Unaroyal's treasury promoting her entire Garrison to Field Units before disbanding herself, to deny Gobwin Knob the benefits of decrypting her Units, or claiming her Treasury
It likely costs more to pop a unit as a Garrison unit, then promote it to a Field Unit, than to pop one as a Field Unit initially. If not, then it may be a way to build a cheaper army.
Based on the importance of the Capital city, it can be assumed that a unit that exists only as a Field Unit would generally be weaker than a Garrison Unit, though some exceptions obviously exist (Stanley's Dwagons as a major example)
Another possible difference is a field unit might cost more shmuckers in upkeep than a garrison unit.