Some contractors prefer to purchase the materials for you for 2 reasons... 1. they get it at a discount they do not pass onto you.  2. they may purchase a better quality product for you.  3. some items have many products that are similar to you but not to the installer.  They may chose to purchase the one they find easiest to work with.

Other Contractors will complete the job with your materials on hand, but do not expect to save money unless you are in the trade.