Sometimes people think that the spouse who earns less money outside the home should do more of the housework. It might be thought that the less-earning spouse is "working less" and therefore ought to make up the difference by picking up most or all of the housework.
However, this analysis fails to understand that effort is not always proportionally compensated. A partner in school, raising children at home, or dealing with illness might be working just as much as someone at a paid job. Someone in an unpleasant low-level job might be expending as much effort as someone in a high-powered career. It is difficult if not impossible to compare people's levels of effort; and it is unfair to judge how much people are working based on what they are paid.
Looking at earning power alone, it might be argued that the less-earning spouse's time is less "valuable" if they make less money per hour. However, earning a lot per hour is not an excuse to throw all the housework onto the other partner. If anything, it's a reason to hire outside housekeeping help, if one's earnings per hour saved by not doing housework justify the expense.
Most of all, it's important for equality in a relationship to maintain the equality of both partners' time. How can a relationship be equal if a guiding assumption is that one partner's time is less valuable than the other's?
Sharing housework equitably is a great way to establish and reinforce the equality of both partners in a relationship. This practice says, "We are equally valuable. Neither of us is too special for housework. If housework would interfere with our other work too much, we do the honorable thing and hire outside assistance.
Read Benefits of Sharing Housework Equally for specific reasons that splitting housework helps couples.