Why should I use virtual memory ?

Hello experts.

I'm SAP beginner.

Now I'm reading some paper about memory parameter.

I found the physical memory is distributed to OS and DB, but roll area, extend memory, private memory is work on virtual memory.

I aleady know that physical memory is faster and more expensive than virtual memory (HDD).

But when I read that paper, private memory and extend memory were not that huge.

I was just about 8GB ~ 10GB. I thought it was not expensive ! Just a person can buy it.

So I confused.

Is there any difference between physical memory and virtual memory ?

I don't know the reason that I have to use vitual memory for private memory, extend memory, roll area.

Can you explain to me or introduce the SAP notes about this issue ?

Thank you

Regards,

Oh