Sounds a bit crazy, but the reason is the paint forms a seal around the lid.  This will help the paint last longer when you store it.  Just make sure to place a mat, plastic wrap, cardboard or drop cloth on the bottom in case it leaks a bit (which it might).  Also, storing them in the garage isn’t necessarily the best place due the amount of heat we get in San Antonio.  I always make sure I have left over paint for clients after we complete their home remodel or room addition so they can do their own touch up painting.  I also use a Sharpie marker and label the paint can with the date and where the paint was used.  Example:  Garcia Home Remodel, 78209, 2/23/2010 kitchen wall color.  This will help in case you use several different colors in different rooms.