zeyuanfu :
Oh... That's what's RAM for, is it?
Similar but different. The RAM holds the data needed by the CPU or GPU until it is no longer needed, but computers don't think like people do. For example you might have 32GB of RAM, and a game that is only 8GB counting everything. First, it will need to decompress the textures and other parts of the game, that might well double the size of the game to being 16GB.
Now to us, it makes sense to think the RAM can hold all of these files so lets go ahead and load every file up or half of the files up into the RAM so we don't have to worry about the HDD/SSD anymore, cause that makes sense. Computers don't do this however. When you start a game, it goes and grabs only very specific files and pulls them into RAM, and it doesn't load the rest of the game. It only grabs files on a need bases, so only when you trigger things in the game that tell it to load this town or this building, otherwise it doesn't bother.
On top of that, if it did go ahead and pull everything in and decompress and have it ready to go, the game would start extremely slow. A game like Total War might well expand into over 40GB! The CPU needs to work fast to decompress it which takes quite a bit of time, and the storage device would take a long time to just load it up (think how long it will take to transfer 20GB from your HDD to another one?). It would take at least 5 minutes to start a game like this, limited by the speed of the storage device or the CPU if the CPU isn't fast enough. So it just doesn't quite work to try and load everything at once.
This is why people like me who use programs to actively monitor the amount of RAM inside of my system being used while playing games typically never see more RAM being used than about 6 or 7GB at most, despite having 16GB total in the system.