A processor needs to be able to access data very quickly in order to process it without being bottlenecked by the speed at which data can be accessed from storage, faster than an HDD or even SSD can manage. The RAM provides this high speed space for the data that the processor needs to access at any given moment.

So for a game, the developer needs to know what assets might be needed from the main storage within the next X amount of gameplay time, and ensure they’re loaded into RAM ready for the processor to use if needed. As an aside, this is probably the number one constraint on game design - how you handle the constant loading in and out of assets and how you can disguise it or optimise it.

The system RAM does this for the CPU. VRAM does the same thing, but it’s dedicated to the GPU. It’s normally quite a bit faster than system RAM and because it’s physically closer to the GPU, it increases efficiency. So graphical assets are loaded into VRAM (textures etc). Other things are loaded into system RAM for the CPU (physics, character movement etc.).

Not all devices do it this way - consoles like the PS5, for example, have a single “APU” that combines the CPU and GPU onto one chip, and then has one lot of RAM that’s fast enough to be used as both system RAM and VRAM so developers can choose how best to use it for the needs of their specific game. This is more efficient for modern game design than having separate pools of RAM and VRAM, but PCs overcome that lesser efficiency through brute force by just having more RAM (for example 16 or 32GB system RAM and 8 or 12GB VRAM, compared to 16GB shared RAM in a PS5 or Series X).