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.