Juust like the title states, what is the ratchet used for on the main sheet block?

It takes a portion of the line load off your arm. In higher wind conditions, where you are on the edge of swimming, you probably don't want to cleat, especially when reaching. If you were to hold all the line load, you better have endurance in your arms. The ratchet holds a portion of the load, while giving you instantaneous trimming.
I think the serious racers, who are looking for every extra boat length they can gain over competitors would use the feature a great deal. They are constantly trimming to every small change in wind. The ratchet saves their arms.
The newer models allow you to modify the release load.
The longer your missions are, the more you will enjoy the ratchets.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

The other feature the ratchet provides is that in very light winds you can turn the ratchet off the main sheet will pay out much quicker and easier, and if your jib blocks have a ratchet also turn them off in light winds and you will be able to tack better as the jib will travel to the new side easier and quicker without the line having friction.

You should be able to turn off the ratchet fairly easily and feel the difference between having ratchet engaged and not engaged. The autoratcheting blocks are not as easy to turn off, but the traditional ratches have a little lever to all the freewheel. In moderate to heavy air the ratchet will save you arms a lot of effort.

--
Scott,
‘92 H18 w/SX wings
‘95 Hobie Funseeker 12 (Holder 12)
‘96/‘01/‘14 Hobie Waves
--

Glad to know that. Next time I go out I will try it.
Thanks for the replies.