Why? Because if it ever became one or the other's responsibility through illness, accident, death or divorce, then they will already KNOW how to do it, when to do it and the why behind it all, because they HAVE done it. Specially if there are children BOTH parents need to know what it takes to keep their lives as normal as possible. That's my take on it.
For the same reason older children need to help out around the home on a regular basis so that they can manage when they move on and move out.They can then look after their home too.
Guys want to know why the ladies tend to get the custody? Its because for the most part they are the ones already doing it, some with little or no support from the male adult in the team.