What is reason for valve
The previous owner was using it as either an improvised water tempering device, or to use the cold water line for a hot water circulating pump. 

Either situation only makes sense if the ball valve has an internal check valve. A check valve only allows water to flow in one direction. It will probably say that it is a check valve on the ball valve, also showing an arrow showing which way it allows water to flow. 

If it only allows flow towards the hot line, then it's a improvised water tempering device. If you only open the valve a little bit, you can make your hot water a little less hot to prevent scalding.

If it only allows flow towards the cold line, then it's for the hot water circulating pump. You can circulate the hot water so that you don't have to run the hot water for a long time for it to get hot, and often people use the cold water line for the return loop back to the water heater.