Flat-tappet cams usually are cast iron and have been made the same way for the last 50 years. Most of the time, if your camshaft is a stock with flat tappet hydraulic or solid lifters, an iron gear will work.
Roller cams are generally machined from billet steel. This requires the distributor to be fit with a bronze gear. There are quite a few exceptions with the roller cams. Crane Cams have been making billet roller camshafts with a cast distributor drive gear to allow for the factory iron gear to be used. This is a great feature if you know about it.
Many people buy camshafts from swap meets with out knowing exactly what it is. This is when you can get into trouble by having the wrong gear on the distributor. If either the bronze gear is used on a cast cam, or a iron gear on a billet cam, the two gears will wear quickly.