To initialize them to some number to prevent comparisons between a number and NaN. Typically we use zero for that, but that would mess up the comparisons. So this uses the highest possible int so comparisons will still work ok.

Is there a benefit to doing this?

Hi.
In general, the backslash is used to escape the suceeding character (such as \n or quote characters inside a string).
In this case, it allows line breaks inside a macro definition, making the code more readable.

https://math.stackexchange.com/questions/2547122/sum-of-cells-in-chessboard