#5: So when your entire family comes to visit you, no one has to share.
#4: It's another way to keep your family history. Every time you look in your linen closet you say, "Oh, I remember when we bought that set!"
#3: No matter what color you decide to redo your bathroom, you probably already have a set in the color you need.
#2: Okay, can't really think of a #2 reason.
And, now, drumroll please . . . the #1 reason to keep all your extra towels . . . so when your 4-year-old son decides to FLOOD your downstairs bathroom (I could've gone swimming, folks), you have enough towels to absorb the flood plus soak up the water that has seeped into surrounding carpeted areas!