Yup, just said above. Let me give you an example.
Case 1) You have two arms but a single big box of candy with a tiny opening, you can only put one arm inside at a time to get the candy
Case 2) You have two arms and two a little smaller(with the same amount of candy as the first single one) boxes with the same opening, so you can put your both arms to work simultaneously , therefore you get the candy faster.
As a very-rough example that's the difference in question with your RAM.
While you can use one, two will make things faster/easier for your PC to handle.