A DC outlet?
Yea, better amp makers will add a 9v out from their amps for pedals.
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That's not why they did it. I don't remember why, but it can't be for pedals as the practice was common in the 60's, nearly two decades before pedals started using anything but hard wired power cords or batteries. Fender did it for a long time and Mesa as a higher gain brother to Fender (at least the Mark series) used them too. Mesa used them on the Mark III but not after that as far as I know.