Why is the inspectors entrance at the end if Act one so dramatic?

It isn't quite the end of Act 1 but it is dramatic as Birling sets out on another long sermon of sound advice, uttering ideas central to his philosophy:

“the way some of these cranks talk and write now, you’d think everybody has to look after everybody else, as if we were all mixed up together like bees in a hive—community and all that nonsense. But take my word for it ... that a man has to mind his own business and look after himself and his own—and—”

This theme inequality among people and the callous sense entitlement of the upper-class is at the heart of the play.