Remathilis
Legend
They aren't going outside the book, they are breaking down the players portion and the DMs portion.They are pulling out a lot of information that players need to play the game and sticking it in the DMG. These are more instances that show the PHB to be an incomplete book. Players shouldn't have to go outside of the PHB to have basic knowledge of how things like stealth, social skills, searching, etc. work.
Those things will influence what kind of characters people will create.
The player knows exactly what to do, make a DC 15 stealth check when out of sight. The DM knows a.) whether the conditions are right to do that and b.) what the rules are for being spotted. The exploration rules are vague, which implies to me they will be expanded upon in the DMG since exploration is more of a DM facing system than a player one.
I could be wrong. No one has seen the DMG. But I think they tried to move parts of the system that require more DM input into the DMG. We'll see in November...