Phasers are much more effective against shields than torpedoes are. If against a ship's shields a phaser hit and a torpedo hit do about the same damage, wouldn't you rather use the one you can keep spamming?
In the first Star Trek reboot (2009), during the kobayashi-maru simulation <PERSON> targeted the enemy ships with torpedoes, knowing that their shields were down because of a hack he put in the program.
The shield absorbs, blocks, and reflects most of the energy and redistributes it across its whole. If any energy gets through then its something the shield normally wouldnt block anyway (eg, certain kinds of radiation, and shots which are modulated to bypass and match the shield's frequency). Any damage the ship takes from a hit to the shields is probably buffeting from the Shield pushing on the Shield Emitters which then transfers some of that energy into the ship superstructure (sorta like blunt force trauma on a ship level).
Against an un-shielded ship, Phasers do decent damage to a ship by themselves but a Torpedo hits so strong it can severely cripple or destroy the ship in one shot. Of course this depends on the size of the ship - A Borg Assimilation Cube is 6000 meters size per side, and it regenerates, but at least it doesn't have shields.
(though it makes me wonder why not just make the Warp Capable Torpedoes hit the borg ship At Warp Speed or something close to C... then the explosive power of it pales in comparison to the kinetic striking power. But oh well).
Torpedoes can also be useful if you're having problems hitting the target, but I think using the torpedoes in this way is unrealistic and was used for theatric performance in the show. 898
The idea of firing a spread of torpedoes would be to get at least one of the torpedoes close enough to the offender so that it gets hit with an energy-shockwave.
But... if it can take more than one shot to blow up a de-shielded ship on a Direct Hit (or even with an internal explosion in some cases) then we aren't exactly talking about NUCLEAR YIELDS here, contrary to what some star trek theorists claim.
If a torpedo is listed as having a 25 Isoton Yield then im saying thats probably more like 25 Tons of modern High Explosives. Thats like 10 modern day cruise missiles in one hit, and the torpedo itself is a fraction of its size. An admirably strong weapon for a warhead you can probably carry in a suitcase. But its not a briefcase nuke by any measure. Its somewhere in that gray area between conventional explosives and nuclear.
Yes it might use antimatter, but the amount of it is miniscule or even molecular, contained in a number of little magnetic bottles "cells", which it ends up reacting with (1:1 ratio matter decomposition) whenever they get broken. Personally i'd think the amounts involved would be extremely small, for safety reasons, so that the containment modules could be overbuilt to make sure that they dont lose containment or end up with a torpedo that explodes when you accelerate fire it out the torpedo launcher.
In which case the enormously powerful Quantum Torpedoes developed later on are each hitting with a strength comparable to a tactical nuke (300 Isotons = 300 Tons of TNT = abut 1/3 of a Kiloton).
Another reason I say torpedoes don't detonate in the nuclear range is because they've been used on planetary targets a bunch of times and they only cause localized damage, not a huge city-killing blast. If a torpedo can hit somewhere close by and not kill the away team, its most certainly not a nuclear weapon, not even in the kiloton range, much less in the megaton range. 898