Answer:
Because it has to check the disk that Windows is installed on.
Explanation:
The chkdsk program needs to check all structures on the disk, including ones the Operating System would be using if it was running on that disk. Therefore to check the disk that Windows is installed on chkdsk must run before Windows is started, so that those structures are not in use.
Chkdsk command to use in a command prompt window that doesn't require a  restart to run is "Chkdsk C:"
