Those -1 objects are translating an integer to a umenu index number (which counts from zero). So they are preventing a modulo error. But I don't why I used a umenu in the first place. I think it's because I used part of another abstraction I made which included a umenu. The truth is, you don' need a umenu or even a third number box. All you really need are the top two number boxes. Just remember that it's always the number under [sel 1] that is selected when holding the mouse button down.
I will say though.. regarding pure data.. you will often see [+ 0], [* 1.] or [- 0.] in Max. In those cases, it's to convert floats and integers into one another... but that's a different matter than offsetting or correcting for a zero/modulo error.