you cant see past 24 fps (human natural eye register), but it makes a difference to fast moving objects ... lower fps will make things skip across your screen, so naturally gamers want the highest possible so no skiping occurs. especially useful when youre playing first person shooters and turning on the spot, you dont want your surroundings skipping past you, you might miss an enemy.
vram is where all the data for the display is held. the more of it, the faster it can be displayed and in higher resolution (speed comes form not having to cache from ram/page).
think about 256 colours ... its not gonna take much vram for that, what about 16 million colours ... gonna take more right.
same with textures, a flat smooth surface like a skyscraper tower wall will not take much vram data to render, but a mountainside will (all the dirt, trees, possibly animals). vram is a resource, a small amount allows you to see a small amount of detail. large amounts will allow you to see a lot more detail (depending on game settings and what the game will render).
i generalise so that you can have a vague understanding of what fps means and what vram is used for.