A "pagefile.sys" is a hidden file used by Windows and placed in a root of a hard drive that allows the system to free up RAM for applications that are in use. Applications that are minimized or inactive have memory shifted to a pagefile.
Programs that have had some memory allocated to a pagefile take longer to restore and may cause a hard drive to slow down slightly. A computer that has disabled pagefiles is likely to start crashing when all its RAM has been exhausted. As of 2014, certain programs, such as virtual machine or defrag utilities, do not function properly if the pagefile is disabled on the computer.