It's not always feasible to carry around the bass boomwhackers from place to place. The octave caps mean you can have 2 octave-range of boomwhackers to play tunes, or have bass notes without those larger tubes, so each tube is a two-for-one.
I've also found that being able to tap the caps is easier for some younger children. Sometimes it can be a difficult physical task to wave around or swing the boomwhackers, and this provides another way for those with differing physical ability. You can tap them with your hand, or on a carpeted surface. Well worth it.