What is the role of hydrochloric acid in a copper electroplating bath? Considering water as a given, the third most important ingredient in a copper electroplating solution, as a short survey of related recipe webpages will show you, is hydrochloric acid (HCl). Here's a sample link: http://www.thinktink.com/faqs/cupltfaq/cupltf01.htm

I can guess that the role of sulfuric acid (the second most important ingredient; the first being copper sulfate) is to provide you with sufficient anions to allow copper cations to easily enter the solution at the anode. But in almost all recipes that I have seen, a small amount of hydrochloric acid is added. I wanted to know how useful it is and what is the opportunity cost associated with it.
As already mentioned by @GM it indeed is used to take out $\ce{Cu^{2+}}$, but it is also used to take out $\ce{Cu+}$.

I know this because I once used a mixture of $\ce{HCl}$ and $\ce{H2O2}$ to etch copper boards, in that process both $\ce{Cu^{2+}}$ and $\ce{Cu+}$ were dissolved by the formation of $\ce{CuCl2}$ and $\ce{CuCl}$ with $\ce{Cl-}$ from $\ce{HCl}$.