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I seldom have a free hand to use it alot, but there are times when it just HAS to be used to get the right effect. Sliding a note of a steel guitar, bending the pitch of a reed instrument, doing a trombone riiiiiip!
You can also assign the wheel to trigger midi commands, such as vibrato or leslie speed. It's a handy little gadget if you can sneak a hand onto it. Try playing a lead guitar solo without it, and it just won't quite sound right. Many instruments depend on the bending of pitches to "define" their character. Sax, guitar, violin - they are all famous for "playing" in the cracks BETWEEN the white & black notes on a piano keyboard. You need the wheel to get into those cracks.