We could say they’re supposed to add an element of tension, of risk, to the game. Whether the defusing method is D&D’s dice rolls, Elder Scrolls’ minigames, Avernum’s “skill equal to or greater than to some preset number”, or any other procedure, you’re supposed to feel uncertainty about success and worried about failure.
As we know, however, most players in this situation will save the game first (unless they forget, oops). Then if their character is killed, or hit with a nasty condition for which no cure is available, they just reload and try again. Except for “equal or greater”, since you either can or can’t.
While there might be a small feeling of danger, then, there isn’t a lot of it. The risk is actually minimal, because the only consequence of failure is a reload.
With that in mind, why bother with traps at all? Especially the minigame type, where you have to fiddle around in some way to deactivate the thing. Spending several minutes - or possibly more - of effort, only to have it blow up in your face, is certainly worse than the swifter dice role or skill comparison.
I believe that traps really belong to the pen & paper games. There the DM is on hand to make changes on the fly, even fudge results if necessary. Good DMs aren’t out to kill the PCs, although a little torture now and then is good for the players’ souls ;)
Of course, a trap need not be immediately lethal. However, if I were some evil being protecting my home/treasures, I’d certainly want all my traps to be as deadly as possible. Otherwise, why have them at all?
So what if designers dispensed with traps altogether? Would that make the dungeon/lair/whatever seem less “real”? Would it make the game seem too easy, even though we know we’d be able to reload had there been traps? Would you really miss them? I don’t think I would.