I would say that the biggest positive to things like a charm pack is that they include all the fabrics from a line. Many Quilt Shops will not buy the whole line. They often pick the fabrics from the line that they think will sell in their shop. They will likely have half the line or even three-quarters of it. Even a shop that does buy the whole line from a company will sell out on some of the fabrics in that line and now you couldn't get everything at that store.
Picking up charm packs, jelly rolls... gives you huge number of coordinating fabrics. Five or Six charm packs will make a fairly large quilt in a short time.
Bottom line, they ofter a large variety of coordinating fabrics in a single package. My 2¢
There is also the down side of having the whole line in a charm pack. Rarely do I like ALL the fabrics in a line. There always seems to be one or two clunkers in there. With a charm pack, you'll now own some clunkers that you'll hate to waste and hate to use.