You can cripple your opponent by giving them a useless item (though things like Flame Orb would usually be better).
Like sumwun said, they're allowed as held items in competitive matches in the games, so they would probably have to have them in Showdown as well. The same thing happens with TRs and Sweets, items that seemingly have no use.
Plus, in Challenge Cup 1v1 formats, they're some of the many items that can be randomly generated for your Pokemon.
Hope I helped!