Since Beta 10.1, Moldbygg is a taunt card with the added ability of completely healing itself and ally Moldsmal, Moldessa and even other Moldbygg at the start of your turn. This makes it particularly useful when you combo it with Toriel on a full enemy board, as it gives the opponent only a max ATK of 4 and can't place another monster to buff that. The opponent has to rely on their spell cards then.
And with the Feast buff of Beta 9.3, these 3 Mold monsters are extemely good in a Kindness deck. This is because Feast will bring Moldsmal and Moldessa up to 6 HP and Moldbygg to 9 HP. Moldbygg will also fully heal all of them at the end of your turn.
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.