Men are better equipped biologically to handle risk, disappointment, physical strain, workplace "drama", and long hours better than women (generally of course), all the things that come associated with providing substance for a family. Women are better equipped to be nurturers, be more in tune to the needs of others, handle prolonged emotional stress, make more careful and calculated decisions, and have better memories in general, all the things associated with tending to a home environment.
Does this mean that it's the way things need to be? Of course not, but it certainly is more ideal.
I have no ideals. If a family decides that the man be the breadwinner, then so be it. If that is what works for them. If they decide that the woman be the breadwinner, then so be it if that is what works for them. The thing is, not all men are cut out to be breadwinners. Some might be more suited to stay at home with the children (if they have any) and not feel emasculated because of it. It all depends on the individual. There is no law for or against it. If this is the way a family has decided to do it, great and I will not condemn them for that even though that is not the way I would want my household to run.
Both men and some have tasks to do in their lives. While some tasks may differ, both can always be the main breadwinner for their family. Sometimes, if you have single parents, the parent can also be the one that works for their kids. There should be no separation between men and women.