Windows, by default, uses processor and i/o idle time to eagerly write memory pages to the hard drive paging file. This allows memory to be more quickly swapped out in favor of other, more important memory when you do things like switch between programs or start a new program.
It's a scheme that was set up in the days before laptops and battery life considerations. Most of the time it's still a net win. If your drive were still spinning there'd be almost no downside, and it's hard to understate how much this helps performance. Sometimes (like when your drive has been spun down for a while) there might be battery-life advantages to waiting until the disk needs to spin up anyway, and writing as soon as it starts to be idle again. However, this would come at the cost of potential performance.