Your society's bylaws provide the underlying structure that supports the way your society is supposed to work. Think of it as your society's skeleton.
But to be truly effective, your society needs more than a set of bylaws. It also needs a policies and procedures manual. Such a manual outlines the way your society functions in more detail. It's like the muscles that attach to the skeleton and that do the real work.
For instance, your policies and procedures manual might give the details as to how you conduct a board meeting (especially if you like to do it a little bit different from the way it's done in Roberts Rules). It might set a maximum value for how much your program chair can spend on a monthly program before having to ask the board for permission to spend more. It might give a set of steps for finding people to run for office (e-mailing them, making announcements at monthly meetings, etc.).
If you write all of this down in a manual (and these days, it can be purely electronic, maybe stored as a PDF file), then it can be copied and given to all of your officers. You can even put it in a members-only area of your society's website so that everyone in the society knows the details of how your society runs.
Without a P&P manual, you may waste a lot of time looking up things in your archive of minutes, trying to remember how you did something the previous year. With a P&P manual, you can feel confident that you're acting consistently, month after month, year after year.