The thought process behind it is this: Since your PS4 is in rest mode, it doesn't need to be running background apps or using other network features. Rest mode, for the most part, appears to download games quicker. It's all anecdotal, but most people will tell you that there is a noticeable difference between download speeds in rest mode versus while the console is on. This is just something that a good portion of PlayStation users have noticed. This isn't exactly something that Sony advertises like, 'Hey! Put your console in rest mode to download games faster.' It's not a feature in that regard.