Well, surrealists like to clutter their apartments with random things like giraffes and clocks. One interpretation is just that they need to hold the giraffe so it doesn't get in the way of the lightbulb. They also need to move a ladder to reach the bulb, but the ladder is in a closet, and the closet door is blocked by all those clocks. The bathtub is just a handy open space to put them in. And they are surrealists, so why not put the clocks in the bathtub?