The T/A will get you through a tough spot when plowing, etc. without having to downshift the main transmission. The shift lever in the forward position is direct drive. Do be aware some models will free wheel, when in the T/A mode, if going down hill.
As someone mentioned in another post concerning T/A's, is to not use it as a brake, if it's free wheeling in the low side.
Very seldom do I use it on my 656, but there are times I have for a short period of time. We've had the tractor for 25 years or so, and so far no problems with it.