Torpedos in 40k are interesting, In lore they seem to run in the department of being both a Close range/knife fight range Finisher that lets much smaller ships kidney punch ships from a much larger tonnage if ignored (and similar weight ships just murder other ships of similar or larger sizes). If you ignore a Ship that smaller then you with Torpedoes TO much, they are more then capable of gutting you. Of course, that requires you Disrespect the escort and your own escort screens haven't kept that Smaller ships from making a ballsy move that last in the first place.
The second most common use for 40k torps seems to be as a weapon that forces your enemy to maneuvered, cutting off their choices. If you fire a spread of torpedoes in an intercept course with their current flight plan, You can basically say "no, You don't get to fly their" Saying No to an enemy is always good, Because it limits choices and forces decision making.
Eldar and torps are interesting, In that torps... seem to kinda suck against eldar? But Eldar seem like they would love torps themselves. Eldar ships are build incredibly fast, incredibly glass cannon, and Throw up E-war Haze everywhere they go, which makes Torps, Which Focus on both disrupting the where of your enemy flight plan and acting as the push knife for a kidney shot, incredibly hard to actually USE on Eldar ships because even if they are actually where you think they are, They are so bullshit athletic they might dance out of the way anyway.
Eldar on the other hand, seem like they would LOVE torps, Because its a weapon that requires you get in close to kill, easier to do if you are covered in Holofields, have Literal future sight and can turn on a dime, Meaning they seem like they would be great for letting small wolf-packs of Eldar ships Fuck up much larger ship types who Can't turn on a dime, And the ability for eldar ships to use their torps to deny the one or two flight paths that their farseers say would have led to a good shot seems invaluable.
