Scratch space is commonly used in scientific computing workstations, and in graphic design programs such as Adobe Photoshop.
Many computer users gradually fill up their primary hard drive with permanent data, slowly reducing the amount of space the scratch disk may take up.
Partitioning off a significant fraction of the boot hard drive and leaving that space empty will ensure a reliable scratch disk.
Hard drive space, on a per-gigabyte basis, is far cheaper than RAM, though performs far slower.
Although dedicating a separate physical drive from the main operating system and software can improve performance, a scratch disk will not match RAM for speed.