The Multi-laser is a varient of Lasgun technology. It boasts a high rate of fire with a good range and decent killing ability. Like its closer compatriot, the Lascannon, it uses huge box like power cells which cannot be recharged and must simply be thrown away after use.
They are mounted primarily on Sentinels, part of the Imperial Guard Structure and due to the high movability of the walker, they can be quickly deployed where they will do the most damage. They are especially good at hunting lower armour transports. Also, they are effective against horde armies with poor armour, such as Orks and other Imperial Guard forces.
The Eldar have a similar weapon called a Scatter Laser which has a better rate of fire and a similarly more Eldar like design.