T-Max 100 was designed to be a functional replacement for all the applications that Panatomic-X was designed for - the ones that required relatively high contrast and very little grain. It does those things as well or better than Panatomic-X did them.
In order to accomplish that, it employs T-grain technology, which is particularly well suited for those purposes.
Panatomic-X was never intended to be a general purpose film. In addition to its special purpose applications, with its somewhat higher speed, T-Max 100 is better suited for general purpose use.