Google "VRAM" - there are many websites with information and explanation.
For example:
http://pc4u.org/guide-how-much-vram-do-i-need-for-gaming-is-2gb-vram-enough/
However, the answer is that more vram is necessary when video/graphics performance is suffering and nothing else seems to fix it.
I.e., the existing Vram is the bottleneck. But there are other factors and the bottleneck may just occur somewhere else and limit any real gains that otherwise may have been possible.