Robert Hand says about it:
 
"...the Part of Fortune is a primary indicator of prosperity, but it is more than that. It is created out of the longitudes of the Sun, Moon, and Ascendant, and therefore is composed out of the three of the most important places in the chart (only the Midheaven outside of these three is as important) and as a result becomes as important as they are. In modern astrology there is a tendency to regard the Part of Fortune as a minor point in the chart, but this was not the case with ancient astrology.  
 
In ancient astrology the Ascendant is the primary indicator of the physical body and health. The Moon is also a very important secondary indicator about the body. But there is a difference between them. The Ascendant was considered to be the body at its most physical, material level. The Moon was the living aspect of the body, in fact soul, but soul incarnated in a physical body. The Part of Fortune, being composed of both of these along with the Sun, is also an indicator of the living, physical body and its relationships with the physical and social world in which the native lives. Its capacity to signify prosperity comes from this. If the Part of Fortune is in good condition, the relationship between the native and the physical and social world in which he or she lives is one that supports the native and enables the native to live well. If the Part of Fortune is in poor condition, the native has a harder time living in relationship with the world that supports the native. The same can also be said about the relationships of the Part of Fortune and health."  
http://www.accessnewage.com/articles/astro/robhand2.htm