It's a useful number but its not the whole story. Fitting more transistors into a given area lets you put more chips on a wafer which is good economically. An

It's a useful number but its not the whole story. Fitting more transistors into a given area lets you put more chips on a wafer which is good economically. And it correlates with performance but, for example, Intel has traditionally accepted more restrictive design rules in exchange for more performant transistors and that has hurt their effective transistor density even if their individual transistors have been fast.