These will provide you with 1 - 3 star characters from the 3 basic classes (Magus, Sentinel and Warrior). Don't expect to see a rare Eddie fall from one of these, but you can gather them in bulk to help get additional sacrifice material and get some fragments for rare souls.
When you open these, you'll typically find some 1*, 2* and 3* characters. While most of them aren't going to be anything you'll be using all that often, many of these have abilities that are necessary for completing missions later. If you can, we suggest keeping one of each of these characters on hand and leveling them up as you have the resources. While not necessary, it never hurts to have a wide selection of available toons.
Farming for these is best done in the LoL dungeons. Stacks of 99 can be earned pretty quickly going this route. If you need more, they can also be purchased from trooper badges in the trooper store.