A) They often aren't done shopping. Especially with clothing, it's rare that a person gets an item and then walks straight to the register.
B) They are not the only guests on the floor, I have to take care of everyone in my area. True example, I've saved sales when upset guests find me and say I'm the only person they've seen while looking for help in electronics or furniture, and I get them calm and tell them I will have someone meet them in that area. True example, I've often had guests waiting close by for their turn for help, and I've had guests stop me as I walk to meet a guest somewhere. If we (sales floor) aren't there then those guests walk away without buying.
C) I have other duties, such as zoning and reshop, that directly affect guests and guest experience if they are not done. True example, I've often had people shop off my cart because they can't find the size they need and I just haven't had a chance to put that size back on the floor. Me not there means that would have been a lost sale.