It's assumed the players will search the dungeon logically- and there's nothing really wrong with that, if the players skip areas, that's sort of on them. But a few adventures present barriers to keep the party from advancing- a locked door that requires a certain key, a missing bridge, a collapsed section of the tunnel. What makes this point different from point 1 is, the players might find a way to circumvent the obstacle with a lucky die roll, or worse, using resources in the adventure itself. I don't remember the adventure, because this was a long time ago, but I was playing with some friends, and we came upon a portcullis that was too heavy to lift or bend the bars (remember when Strength had a feature just to do this?) and the only way we could see to raise the portcullis was a level on the other side, out of reach. The adventure wanted us to take the long way around by eventually finding a secret door.
But after an early encounter, we discovered a potion of gaseous form, so the magic user cast invisibility on our thief, she turned into a cloud of mist, waited for the potion to end on the other side, then, naked but invisible, crept to the lever and opened the portcullis, allowing us to bypass 25% of the adventure. When we didn't have important information later, the DM said "well, technically, it was your fault for not exploring the lower levels before going through the portcullis".