Regenerating ammo is often used in space shooters that disallow continuous fire with main weapons, often energy based.
This makes the shooting far more tactical as the player must decide when to fire and when to allow their ammo stocks to replenish..
This feature is also used in shooters that utilise a strong energy melee weapon, a weak ranged weapon or sometimes a shield gun.
In most shooters this feature is commonly used for the same reasons as infinite ammo. However, the player cannot fire continuously with their weapon without loosing rate of fire or even sometimes being unable to use some functions of the weapon.
The dispersion pistol in Unreal is good example, even when it's not upgraded both functions are equally usable. When fully upgraded, the weapon's primary fire will reach full power. If the weapon is close to having full ammo, the secondary fire is unaffected.
In the case of Blood 2, some weapons lose function when ammo is depleted; like the alt fire of the doll, both fire modes of the orb and the alt fire of the life leech
In Half-Life this was used in the Hive hand weapon which can fire quickly eight projectiles.