Women's reasons to become surrogate mothers are many and varied. Money might seem to be an obvious motivating factor, but commercial surrogacy is forbidden in most of the developed world. That does not mean that surrogate mothers get no compensation at all. Most often, their expenses will be covered and they might receive a little extra. If you look at surro mom discussion boards on the internet, you will see that most of the women are not in it for the money. Some already have children of their own and see gestational surrogacy as a beautiful way to help other couples have children.
One surrogate mother on a message board I am a member of has carried three pregnancies to term for different gay couples now, because she feels strongly about helping gay men become fathers and knows how difficult their road to parenthood can be. Other surrogate mothers love being pregnant but feel their family is already complete. Most women who consider carrying another couple's baby are now gestational surrogates, meaning that the egg comes either from the intended mother or from an egg donor.
Essentially, these women are the host to an unborn baby for the duration of the pregnancy. And of course, they will be the one to give birth to the baby. The lack of genetic connection might make it easier to be a surrogate mother. There are women who go through a "surrogacy journey", as the matching process, the pregnancy and birth are often called in surrogacy circles, time and time again. For them, it is all about playing a role in fulfilling other peoples' dreams, and making a change in the world.