If you have not already done so, I'd suggest you get a copy of South Bend Lathe's immortal work: "How to Run a Lathe". It will answer a lot of questions you may have, including the uses of a toolpost grinder.
I agree with all that John Oder, Tommy1010, and Tyrone have said about toolpost grinders. The toolpost grinder enables a person to finish grind hardened parts to final diameter. It is NOT for taking off huge amounts of material, but a matter of a few thousandths. Aside from grinding the headstock center in place, a toolpost grinder would find use in finish grinding hardened parts such as a shaft journal to run inside a needle bearing, or perhaps a hardened mandrel. Another use would be the grinding of engine valve seating surfaces. Most engine valves (as used in automobile, motorcycle, tractor, or diesel engines) have hard seating surfaces on the "valve head". These surfaces have to be finished to a series of conical angles, to seat correctly in the valve seats. engine rebuilders and service garages often have a small precision grinding machine built just for this purpose. In bygone days, shops working on various engines did not have the money for the valve grinder, so made do with a lathe. The lathe saw service turning the commutators on starters and generators. Equipped with a "mica undercutter", it could finish the reconditioning of commutators. as a lathe, it made odd parts such as valve guide bushings, and many other odd parts. Equipped with a toolpost grinder, the lathe could grind engine valves to the correct angles.
Toolpost grinders are handy to have, but the odds of needing one very often are not really high. Mine sits in its box for years at a time.