Using pipe lasers inside pipes often ends up with movement of the laser beam. Every drainage contractor somewhere, sometime has experienced this.
This is referred to as Refraction.

Refraction happens due to varying temperature and humidity inside the pipe as the laser light as it passes through it. The layers of air in a pipe can refract a laser beam, causing it to bend and or move around as the laser beam passes through the air and onto the target. Refraction is especially noticeable during hot or humid weather, and is often worse in smaller diameter pipes (e.g. 100mm diameter).
Using a a pipe blower aids in reducing and eliminating the effects of refraction, with the ventilation mixing the air inside the pipe, creating a more uniform airflow for the laser light to pass through.