At about 6 to 8 months of age, watching something leave his hands and fall to the floor is quite exciting for a baby. He also realizes that his actions may spark either a negative or positive reaction from you, ergo he will continue to throw things since it catches your attention.
As your child continues to grow into toddlerhood, his reasons for throwing things may increase. Especially when he reaches 18 months of age, throwing things may be an act of aggression or a means to display/release feelings of anger and frustration.