Using an echo sl as an example
it difficult to explain, think of it as one revolution of the crank. if the freewheel is at the front with 1 revolution of the cranks it will only engage 108 times. if the same freewheel was on the rear and the front cog was larger (for a desirable ratio it will almost certainly will be) with 1 revolution of the cranks the rear freewheel will spin more than one revolution wich in effect will give you more than 108 engagements per revolution of the crank.
but having the freewheel on the rear would require a much larger cog on the front, which would mean less clearance, larger bash ring (more weight) and a longer chain (more weight), there probably is more disadvantages too but i cant think of them right now.