An X value does not exist in hardware. In simulation, an X value can result from two or more sources driving a signal simultaneously, or the stable output of a flip-flop (electronics) not having been reached. In synthesized hardware, however, the actual value of such a signal will be either 0 or 1, but will not be determinable from the circuit's inputs.
Synthesis tools can use don't care values to determine where and how to perform area optimization. A synthesis tool can use don't care values to reduce the circuit sizes of finite state machines, for example.