A couple reasons that can work together or separately for this to happen:
- Heat management
- A normal "machine gun" style gas operated piston is unsafe for this weapon for some reason
One of the reasons modern vehicle mounted weapons like the GAU-8 have multiple barrels is not for any sort of aesthetic reason but for heat management. Even then, the GAU-8 cannot be fired for long periods because it will still overheat. It has a firing rate fixed at 3900 Rounds per minute.
Weapons with high firing rates tend to overheat rather quickly using chemical propulsion.
Modern Infantry machine gun teams must limit their firing rate and often, a spare barrel will be carried for squad automatic weapons. (One example is the m249, a squad support weapon used by the united states military, read about it here if you want )
Chemically propelled firearms will overheat eventually and if they are fired while overheated it can ruin or deform the barrel, rendering the weapon inoperable or unsafe.
If your weapon is a coilgun which uses chemical propulsion to provide an initial burst of speed to projectiles, this might be good enough for you.
Any weapon with such an extreme firing rate will be multi barreled in current technological application partly because of heat, but also because of the second reason.
If your weapon is not using chemical propellants, you cannot use the same standard mechanism for automatically loading the next bullet as a modern machine gun or automatic weapon uses.
Modern automatic weapons use a portion of the gas produced by the burning of chemical propellant to drive a mechanism that automatically ejects an expended cartridge and allows a new one to be fed into the mechanism. This is a really high level overview as there are multiple types of automatic reloading mechanisms (Read here for more on different types of automatic loading in modern firearms)
Automatic weapons which do not use this mechanism to load another round would require another method to load the barrel safely and reliably. One of these could be using multiple barrels. In this way, because the barrel being loaded is not actively expected to fire the next round, there is more time for a mechanism to properly load the projectile. The Gatling gun is one example of this type of weapon and was used in the United states civil war and other conflicts.
Therefore it is safe to say, firing mechanisms may be the reason your weapon has multiple barrels.
For example, if you had a railgun and not a coilgun, you could say you needed multiple barrels to ram the projectile into place adequately.
The key here is the safety and reliability. Machines in real life cannot operate instantly and as a result there is a sort of upper limit on how fast you can safely reload a single barreled weapon. Perhaps the reason your weapon has multiple barrels is because it is the only sensible way to load rounds in a reliable way.