On the topic of scaling, reversible computations are more energy efficient than non-reversible ones, see also the OP. Outputting the original inputs might seem silly and wasteful superficially but if you discarded them (as "heat"), you'd just be back to building a non-reversible, likely much less efficient gate.

c is an actual input variable, it's not a constant. You can set it to be the constant 1 which the article does in one of the examples, but this isn't mandatory.