Rod and reel combos can help save money in most cases. As manufacturers couple these components together, they ensure the buyer stays within their product lines. This means less money spent on marketing to target customer loyalty and familiarity, which in turn cuts the price for the customer.
Some combos make sense for an angler with a lot of gear designed for one particular technique. A fisherman who love spinning, for example, and has a heap of spinning rod tackle and lures and just wants an extra rod to play with on the water, can get a lot of extra mileage and convenience out of another spinning rod and reel combo.
The same can be said for a baitcaster too.