The purpose is so that whatever you make will hold its shape better since a lot of the times fondant alone will not.
To my knowledge, you use 50/50 to mold whatever you want to make.if you use only gumpaste its expensive and it dries very fast. using 50/50 it gives you time to adjust after making the item. only fondant is a bit too soft and takes longer to dry.
I tried the 50/50 today and i noticed that it stays alot smother and you have more time to work with it oppose to working with gumpaste alone.. When i worked with just gum paste i noticed that i would get cracks after it dried..
I prefer the mix for several reasons. 1) the fondant is too soft to hold together when making flowers and figurines. thus # 2) the gumpaste added to my fondant helps my fondant figurines hold their shape! 3) cheaper 4) gumpaste alone can be quite difficult to work with and get smooth of all little creases and cracks, the fondant added into it helps the gumpaste to mold and shape better. HTH!
Thank you everyone for your response I always wondered not knowing much about either product other than its great for making added decorations to a cake. I will only use it to make decorations like the Mickey Mouse clubhouse cake I'm about to make. I want to do a buttercream frosted cake and add the clubhouse pieces to it so I was not sure which item to use.