Why buy ground cinnamon instead of cinnamon sticks?
Different applications, sticks are mainly used when you wish to infuse a cinnamon flavour in a dish, where the spices are removed at the end, a biryani is a good example.  Ground cinnamon is used when the spice is to be left in, or be present throughout a substance, like in cakes for instance.

Ground cinnamon will leave a stronger flavour partly because it is left in and possibly due to the flavour extracting easier due to higher surface area and broken structure.

Cinnamon is notoriously hard to grind smoothly, so people tend to buy both for their respective uses.