In most cases, the decision between a wet bar and a dry bar is dictated by the size of the space, and the accessibility of plumbing, and distance to it.
A wet bar essentially has plumbing hooked up within it. This allows for quick access to drinks while entertaining.
A bar that doesn't require plumbing, which helps lower overall costs. They can join almost any room in the house, needing only a small area to set up your favorite mixers and spirits.