"Mask of Darkness" is at times avoided because a number of commonly played Traps require your opponent to trigger them. This means your opponent can usually avoid the worst result of these cards. But "Mask of Darkness" is useful if you're running Traps that don't require your opponent to trigger them, such as "Conscription".
On the other hand, you can use the fact that your opponent sees what Trap you recovered to your advantage. For example, you can retrieve "Mirror Force", and make your opponent afraid to attack thus stalling, (even if you set a card other than "Mirror Force").
Many Decks need certain Trap cards as this card allows them to be reused.
This card can retrieve a Trap Monster from the Graveyard, letting you summon it again. In particular, this card can be useful in a "Paleozoic" deck, as the archetype is largely comprised of Trap Cards that treat themselves as monsters.