Usually, the 2 sticks are guaranteed to work together as a pair. (Not that this is a big problem normally) They come from the same lot, or batch, or run at the factory supposedly, other than that, there is no real advantage.
THe difference is that the kits are pre tested to ensure that they work together at the same settings - Modules of the same line will still vary slightly and buying 2 seperate modules on different days\weeks of production can result in them not working well together - so buying them in a kit ensures that you are getting 2 modules from the same production and that they have already been tested to perform together. (mainly it is for users that OC their systems and need both (or all 3 if using triple channel) modules will perform equally so that as you OC them the system remains stable - normally the cost is very close to 2 single modules and sometimes even less since you are buying multiples but there are times when 2 individual modules will cost less.
In most cases unless you are pushing your system OCs the 2 individual models from the same brand will be fine - since they are from the same manufacturer but there are occasions when 2 modules will have come from 2 different factory runs and may have slight differences so when you want that added little performance matching it is best to buy them in kits.