From the menu, I take all recipes, and put every ingredient on the grocery list. For items that are used multiple times, like sour cream, I put the ounces out beside it, or tally marks for canned goods. Then on the final list, I put the quantity out to the side. For snacks, I don't write specific things down. The list just says "snacks" and I may pick some different things each month.
It is very helpful to know how long toiletries, cleaners, laundry stuff will last at your house. For toiletries, I put every item on the master list. Before grocery day, I just mark items off the list, if I have a new, unopened item in the closet/pantry. In a rush, though, I can just grab the list and go, knowing that I will have everything I need for the month.
1--Menu/list takes all day--but then it's done for many months.
2--Time, time, time. We cut down the weekly time spent making menu's and lists. Also puts all shopping (for me) on one day, instead of having to work a weekly trip into our schedule. Cuts down on DH's "babysitting" time--not that he minds; but you probably understand what I mean.
3--less money spent
4--less time in the dreaded Wal Mart--which I hate with a passion, and that is the main reason I started doing it this way!
A few times when DH's schedule gets crazy, we either swap grocery day, or he meets me at Aldi after work, and drives the van and kids home, leaving me with his car.
This is all pretty basic. I hope it helps. If you have specific questions, let me know.