Diamond Boy 1 is a very useful card, giving its ally monsters a bonus Armor (1), an ability introduced in Beta 9.1 (which lead to a description change of this card). Despite the info changing, it is functionally the same.
Armor (1) will make all attacks do 1 less damage, so an ATK of 4 will only do 3 damage, leaving cards like Mad Dummy alive.
Combining it with a Taunt Card and Toriel is extremely useful, as it makes the enemy monster be unable to deal any damage.
However, because they now gain Taunt with it, you can't damage the opponent directly unless you kill all the enemies. They also can still rely on their Spell Cards.
But despite that, it can still be useful to save up some money.