We'll if you have missiles loaded along the aft-forward axis of the ship, then you need to usually rotate the whole ship in the direction of the enemy so the missile tips are facing the enemy before firing. With VLs missiles fire vertical from the middle of the ship and can go in any direction they like after launch with no restriction on the bearing of the enemy before firing.
Using VLS also preserves space, and looks less unsightly than things like the typhoon launcher hanging off the front of the ships. Missiles are only a few pixels across, so a a large normal weapon slot could hold multiple launch tubes.
Also having your missile systems closer to the centre of the ship would make them less prone to damage and going offline.
That and it makes sense if missiles or torpedoes are being reloaded in combat, instead of them magically reappearing on the rack the reloading occurs within the ship out of sight.
The only downside is they have a slower initial velocity? And wouldn't want the engines to burn out before they were clear of the top of your ship.