You'll be wearing the amount of weight required to achieve neutral buoyancy at the surface. But as you descend, your wetsuit will compress. This decreases your volume, and results in a loss of buoyancy.

To maintain neutral buoyancy, you need to add air to your BCD as you descend. The principle behind this procedure is that as volume is lost in your wetsuit, the same amount of volume should be added to the BCD. As you continue to descend, the air in your BCD also compresses, so you'll be compensating for volume changes in the BCD as well.

You do the opposite on ascent. As you ascend, your wetsuit in addition to the air in your BCD expands, so you need to release air from your BCD to maintain neutral buoyancy. This prevents an uncontrolled ascent.