Man in an apartment building

here's a pretty old one. There was a man. He lived on the 24th floor of an apartment building. Usually, when he went to work. He took the elevator down to the lobby and walked out of the building. When he returned, He took the elevator to the 16 floor, and walked the rest of the way. On rainy days, he would go to work the same way, but when he returned home, he took the elevator all the way to the 24 floor. Why?