It's more a personal habit, I just like to return things back to where they were after use. But I did leave discs in players before for a long time and they get coated with, I don't know dust or vapor (like if you breath on a cd) or what, but need to wipe off. Granted those are mostly normal tray drive cd/dvd players, not an enclosed slot drive like the PS4 where dust is less likely to get in. I did used to leave my Dragon's Dogma disc in PS3, which is also slot drive, and had to wipe it after months. There's also, probably irrational, fear of things like heat (in an enclosed unit that heats up, cools down, heats up), moisture (moisture plus heat), and maybe will scratch the disc if move the console, or the disc will scratch the laser head if moved, and so on. Well, no real case of actual damage, just preference I guess.