Cos the "ing" makes it an action word... BAM ! I'm so good at grammering.
Why do we use ing form after the word guide? For instance: a guide for making coffee. Why making?
This makes sense:
"Surely the infinitive to make is a clearer marker of purpose than the ing-form."
http://english.stackexchange.com/questions/241456/is-to-make-sure-vs-is-making-sure
In other words, "to make" implies there is only one way "right" way to make coffee; while making implies there are several ways from which you can choose.
I find that to be a useful distinction.