I can play in just about any key, but I'm most comfortable in the key of "C", therefore the transpose button allows me to transpose to a key that is the comfortable for my vocal ability.
All the best,
Gary

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PSR-S950, TC Helicon Harmony-M, Digitech VR, Samson Q7, Sennheiser E855, Custom Console, and lots of other silly stuff!
K+E=W (Knowledge Plus Experience = Wisdom.)