What is the genetic purpose to women having thicker legs and why am I so attracted to it?

Thicker legs support wider hips and wider hips are necessary for giving birth. But let's be honest, this "thicc" thing has kind of verged into overweight territory and it's not entirely healthy. But if you wonder why women need strong thigh muscles...you try carrying a nine month pregnancy around for a while and you tell me. 

No such thing as genetic purpose. Evolution has no purpose; natural selection just favors some mutations, and weeds out others.

Perhaps big thigh muscles were an advantage for our nomadic ancestors who walked a lot.

To hold up a fat @ss.

Womens strength is in their legs, while mens are in their arms ...thus handed down. I suppose you find it beautiful as it is differ from mens legs. 

Especially in young women this extra fat on the inner thigh is very exciting if expertly flashed now and then. It helps to attract partners and pregnancy. Unfortunately, many women are made unhappy when they let their guard drop and put on lots of weight, and the lure of this enticing fat is lost as it becomes thicker. Sensible women find out that they can banish the bad effects by jogging and eating well and some have shown that they can maintain their allure, yea, even unto their seventh decade.

I never did my research on this, but it may have to do with fat supplies for giving energy to a baby, when pregnant