Moldbygg is a cheap card decent for quick card placement, making it useful in Bravery. Since Beta 22.0, Moldbygg is a taunt card with the added ability of giving +1 HP to all ally Mold monsters on the board, excluding itself and other Moldbyggs.
This makes it particularly useful when you combo it with Toriel on a full enemy board, so the enemy is forced to use spells or waste about 2-3 turns killing this. Each Mold also works very well alongside Feast in a Kindness deck.
And as said before, it's also good in Bravery, especially when Strength is used to not only do much more damage to enemies, but also prevent it from dying by Muffet's Pet. And when given more HP with Migospel, it will become really hard to kill Moldbygg.