From UFStarfleet Wiki
The mounted phase cannon is a relatively new technological advancement. It was first used on experimental Galaxy class starships known as the [Galaxy Mk III]'s in the late 2370s. It has since become more common among newer vessels like the Sovereign class, Redeemer class, among others.
Pros and Cons
The mounted phase cannon was designed for one purpose: tanking. A ship with this weapon equipped would be able to unleash one or two extremely high-yield phaser hits on a target. This is helpful when dealing with strong, heavily fortified space stations or starships.
However, this takes up a lot of energy and often drains the entire ship's phaser-banks for a short time. It also hinders warp capabilities for a relatively significant period of time, depending on the ship.
Also, due to being mounted onto the hull itself, it can only fire straight ahead. No targeting or aiming is done either manually or automatically. This means that the target has to be slow and/or large. The target must be directly ahead of the ship when the beam is fired, or a lot of energy is wasted.