You can take some space from HDD and save it for a cache of sorts. That is virtual memory.
It is however very slow compared to ''real''memory so you won't notice any difference. Personally I've kept around 4GB's of virtual memory since the last 4 or 5 years simply because I've very conservative of HDD. I very rarely use more than 100GB so I think it's a waste not to have some virtual memory
Virtual memory is almost a leftover from the days when memory cost $50+ a megabyte. Your OS will reserve a certain amount of hard drive space and treat it like memory. This was important when the average computer had like 4 megabytes, or even maybe in later years, a gig of memory.
Today though, with most systems now using 4gig or more, you don't really need this function even turned on. You should reserve a small amount though, as the OS tries to save a log to virtual memory in the event of a crash, and actually I don't know or have ever even heard of anyone saying this was really useful for anything.
Anyway, if you have 4 gig of memory using a 32 bit OS, or 8+ gig using a 64 bit OS, you can simply disable, or set this to a very minimal amount, like 50 megabytes, really, most people simply don't need more than that, for any reason.