Infantry and static get the most benefit from fortification. Since fortification is lost when a unit is moved or put on front line attack, this makes them ideal (IMHO) as garrison units. Drop a formation of infantry and static with a few construction vehicles to add fortification on a colony planet, let it sit around building up fortification for years, and if that planet ever gets invaded they're going to be much tougher than they would normally be for their cost.
If you just drop troops on a planet, though, they have no fortification. This is dangerous for infantry (with 60% hit modifier) and suicidal for static (with 100%). Vehicles, on the other hand, have decent hit modifiers but lower maximum fortification, which makes them decent on the offense.