MemTest is a RAM tester that runs under Windows. It verifies that your computer can reliably store and retrieve data from memory. A correctly functioning computer should be able to do this with 100% accuracy day in and day out. A computer that fails these tests, perhaps because of old hardware, damaged hardware, or poorly configured hardware, will be less stable and crash more often. Even worse, it will become even less stable over time as corrupted data is written to your hard disk.
By running MemTest you can ensure that your computer's RAM correctly functions. It is wise to test this when you buy a new computer, install new RAM, or change the configuration of your machine (for instance, to overclock it). Are you the sort of user who likes to push the performance of your machine to the edge? Relying upon whether your machine will boot after your new BIOS tweaks is a poor way to determine the safety of your new settings. Use Mem Test as a true test of stability.