What does a ignition cleaning do for my car

Say what? No such thing. Do you mean a fuel injector cleaning? Is your car dealer trying to fiance his second vacation home?

Cleaner ignition means better electrical conductivity. But never heard about cleaning an ignition on a car before unless someone deliberately putting dirt, mud or anything to prevent you from starting. Fire that mechanic of yours.

I would think F_O_R is correct in that you are meaning injector cleaning. If the vehicle has high mileage like over 100,000 then an injector cleaning service may be beneficial. Similar results may be had by using top tier gasoline and adding a bottle of Techron fuel system cleaner at every oil change. Do a "Google" search for "top tier gasoline". Techron is available at auto parts stores and Walmart.