The skin material on the feet of TPE and silicone models is soft, and standing a doll directly on this material without a "Standing Feet" option will damage the appearance of the feet. Dolls made with "Standing Feet" don't just have foot bolts, but they also have a different "foot plate" attached to the internal skeleton which is larger and allows the bolts to be held in place to transfer weight from the frame directly to the ground. 

What are Standing Feet?

 

Essentially, standing feet are three weight bearing points that are on the bottom of the models feet to ensure weight distribution from the skeleton direct to the floor (without putting force on the soft TPE or silicone). They also offer more stability when they are in a standing position. 

Hint!

Try to keep socks and / or shoes on the feet while standing the model as this will keep the feet clean. A doll without standing feet can also be *attempted* to be stood up if it is wearing tight-fitting shoes, but some damage is still possible to the internal soft TPE and silicone. You always need a three point stance (meaning a wall or some sort of structure to lean on) to allow the model to actually "stand"