Increasing waste capacity

Allison was brushing her hair in the bathroom.  Behind her there were two garbage cans.  We only need one... Why were there two?   When we were taking out the garbage, she took the garbage can from her office, emptied it in the garage, and put it back in the bathroom, right next to the bathroom garbage can, unaware of what she was doing.  Then she turned around and started brushing her hair.