These are EV enhancing items that are really useful when Ev training.
>EV-enhancing items are items which enhance the EVs gained when a Pokémon is defeated. All of them halve the Speed of the Pokémon holding them in battle while they are being held.
These items' effects are doubled by the effect of Pokérus. For example, if a Chikorita with Pokérus that holds a Power Lens defeats a Haunter, that Chikorita would gain 12 EV points in Special Attack. Two would be from defeating the Haunter, four would come from the Power Lens, and the remaining six would be from the Pokérus's doubling effect. However, a Pokémon holding the Exp. Share will not gain any of these boosts, and unless already infected with Pokérus, will only receive the base EV yield of the defeated Pokémon.
From Pokémon Platinum onward, if they are held by either parent in the Pokémon Day Care, that parent passes down its IV in that respective stat; HP for Power Weight, Attack for Power Bracer, and so on.- Bulbapdeia
They also half the users speed and cause the Pokemon to dislike the user a little.