Suppose a sci-fi universe with the following constraints:
1. Space dust and rocks etc. are deflected with some sort of strong force field; these are your standard "deflector shields". Possibly there has been an escalation of arms between deflector shields and plasma cannons, which will not be hugely material here except that a "deflector shield bank" might be a valid target for trying to selectively disable.
2. Lasers work much as they do in the real-world: invisible, they travel at speed c, they can't be deflected by deflector shields really. They have, however, been neutralized by dissipative hull technology, which requires a lot of mass but basically means that you need to focus a laser on a point for several seconds before you can melt anything.
3. This is a problem because our mastery of some non-Einsteinian notion of gravity has led to inertial dampeners which allow massive ships to have a sort of characteristic "jitter" which distributes the laser over larger areas. It also makes it harder to target small fighters or large cruisers with projectiles, so even though we have some sort of mass drivers (maybe railguns, maybe not) firing masses with high speeds, this is not enough to hit a ship with a precision payload.
4. You have some ability to see "hot" projectiles coming, whereas you would not be able to see the normal bullets from a mass driver.
5. Electromagnetic pulse weapons can disable electronics at close range.
6. There is some distinction between small, agile "fighters" and medium-size "destroyers" and large "cruisers".
Then it might make sense to have about three different sorts of powered projectiles.
A missile would have an explosive payload, an internal engine, sensors, and a tracking computer. These would be useful for a cruiser trying to get rid of small fighters. 898
The idea is that these could be like "really fast computer-guided kamikaze fighters", you neutralize the EMP threat by always firing a cloud of 20 of them per fighter, so that a fighter gets overwhelmed by trying to dodge these ones while EMPing those ones. With a couple big cuts you can imagine that a small fighter's engines or weapons could be compromised and they're basically out of combat.
A rocket would not be guided at all and would be analogous to a "space bullet." They would be fired cold with a mass driver, then would rapidly accelerate while 'hot,' perhaps by detonating a big nuclear explosion in a blast chamber and riding out the shock wave as it destroys the chamber. They would basically be a big, heavy slug of matter, not deflected much by a deflector shield, targeted only by being fired "cold" one way and then after a timer expires, being accelerated in another (possibly different) way. They probably come in two varieties: "light" rockets are best for fighter-on-fighter dog-fighting combat; they are fast straight-line projectiles not unlike guns, but probably fired in "banks" so that a fighter needs to simultaneously dodge several of them at once. However at larger distances these are pretty easily melted by lasers before they hit a deflector shield which will deflect the vaporized forms, so for mid-range cruiser-on-cruiser combat you instead see bigger slugs, basically a big chunk of dissipative hull, so that lasers do not melt enough of them before they pass through the deflector (and then any further laser-ing would just add heat to the remainder of the slug, increasing hull damage).
Finally, maybe torpedos are rockets which are partially guided by the firing ship, therefore they behave a little less like "bullets" and more like they live in some "fluid" medium which they can push against. Imagine that we configure this blast chamber to push slightly off-center from our rocket payload, then as it gets accelerated to high velocity it also spins wildly. It's possible that, since we know the trajectory it's going to follow ahead-of-time, we track that trajectory with a carefully pointed laser. That laser then evaporates some external "casing" on the spinning payload, which gets ejected in some preferential direction. The result is that torpedos appear as "curveballs" for a period of time after they go "hot". They are maneuverable enough to vaporize destroyers but overkill for (and perhaps unable to target) fighters. They have a slight accuracy benefit over rockets for doing precision strikes on those jittery cruisers, since they can be guided somewhat "en route", but probably there is a tradeoff here. Presumably once they are detected and the target's lasers are reoriented to fire at them, they can be deflected from their intended target by laser defense just the same way that they are guided. 898