Consider this, comparing the mechanics):
DM: Make a Perception check.
Player: Crap, a 7.
The floor suddenly drops below you and you fall into a pit, landing on several spikes for 11 piercing damage. Make a Constitution save.
Player: 17
DM: You feel a slight burning, probably poison, but it doesn't cause any other effects.
or
DM: The floor suddenly drops, a pit trap? Roll percentile dice.
Player: Crap, I hope I make it. 37.
DM: You tried to leap free, but the floor dropped out too quickly. Roll a d6 to see if you land on any spikes.
Player: Spikes? Ugh. OK, A 4.
DM: You manage to rotate enough to land between two of the spikes, they glisten slightly with a thick black substance.
Player: Probably poison. Whew, that was close.
The first one is quick and doesn't build any suspense. Also, failure to detect the pit always results in damage. This is the 5e mechanic in the updated ToH.
The second one is the original mechanic. It builds more suspense, especially since players know that poison is save or die. Now they know there are pit traps with poison.