The reason that this is important is that degenerative diseases cause the brain to shrink. This could help to invalidate Broca's claims about the results of his tests.
Broca claimed that his study showed that women have smaller brains than men. This proved that they were mentally inferior to men. But what if the women's brains were smaller because of some other factor other than sex? This would tend to disprove Broca's argument.
Gould argues that such other factors do exist and that degenerative disease is one of them. Many of the women Broca studied were old and, Gould says, this implies they had degenerative diseases that would have reduced the size of their brains. This is important because it offers an explanation (other than sex) for why the women had smaller brains. This helps to undermine Broca's claim that women are intellectually inferior to men.