Why do poured concrete walls have holes?

Gas (air) trapped in the mixture was not removed. The bubbles make the holes.

One method to reduce the holes and get a smooth finish when the formwork is removed, is to use a concrete vibrator. This tool is poked into the wet concrete before it sets and vibrates the mix to encourage air to come to the surface. It can be very effective. This also makes the concrete stronger by eliminating weaknesses caused by air pockets.

Concrete vibrators must be used carefully and should not be allowed to contact the formwork or reinforcing as they can mark the surface and move the reinforcing. Excessive use can also separate the fines and the coarse aggregate making the concrete weaker.