Wj
When does an application usually need something from the storage?
First one is at game loading of course but after that is for instance when reading maps or extending them, Paging memory to page file in OS is most common during game play because not all code can fit into RAM itself. OS also pages memory without regard of what else it's doing (like playing games). Depending on program/game GPU and video in general may also do same thing.
Programs/games usually preload all of that in the background so it's not immediately obvious, you can notice it by watching disk activity light on the case. Prolonged steady or fast blinking light is sure sign of much disk activity.