Dips and dip variations are considered lower chest and/or tricep developers. But if done properly they are great for overall chest development. Keeping your knees bent creates a forward vector. Your body wants to move down and forward. To counteract this your clavicular (upper chest) fibers have to really contribute to the movement. Pulling your legs through allows for a better Tricep contraction because your long head has to contribute to shoulder extension in order to pull your hips under you.