Here's why:

* Drainage: When the open side is up, water can drain out through the holes. This helps prevent moisture buildup inside the block, which can lead to mold and mildew growth.

* Strength: Filling the holes with mortar strengthens the block. When the holes are filled from the top, the mortar can distribute pressure more evenly throughout the block.

However, there are some exceptions to this rule:

* Insulation: If you're using insulation in the holes, you'll need to follow the specific instructions for that product. Some insulation might be designed to be installed with the open side down.

* Aesthetic purposes: Sometimes, the holes might be left open on the exposed side of a wall for decorative purposes.

It's always best to check with the manufacturer's instructions for the specific type of cinder block you're using and the type of mortar or insulation you're applying.