While laser pulse rifles can be operated in continuous mode, their normal mode of operation is pulsed, hence the name. The weapons designers discovered that pulsing the laser could be as effective as a continuous beam in killing people. When operated in this manner, they use a fraction of the energy and therefore their power source lasted longer.
The way they work is a quick pulse of laser energy vaporizes the first solid substance it hits. The next pulse drives the "puff" of material forward and simultaneously vaporizes a new section. So instead of the hole being created like you see in the movies, it is more like the proposed Project Orion nuclear spaceship.
After reading The Ark Lords, you may be wondering why the laser pulse rifles did not cauterize the human tissue they drilled through. Conceptually, it is the difference between a dashed line and a solid line. While maybe half of the tissue is cauterized, the rest is blown away by organic material. This is why the people who are hit by laser pulses bleed out so quickly. If the Ark Lords used a continuous laser beam, all it would do would be to punch a tiny hole in the target and therefore not necessarily inflict much damage. Laser pulse rifles are designed to CAUSE massive bleeding.
There is one scene where a soldier does turn his rifle on continuous and swings it around, like a very thin light saber. But mostly they use it to cause massive bleeding and a quick death with very little energy.