What’s it go to do with being trans though?
Well, you should really ask the people who bring intersex conditions up; I probably won't do their reasons justice. But the arguments for why it's relevant I've seen appear to pretty much amount to one or the other of these.
1) Transphobes say sex is objectively binary. But even one intersexed person proves it isn't binary, so they're wrong, so it's not objective, so it's subjective, so it must be whatever the subject feels it is. Therefore transmen are men and transwomen are women.