Unlike the Lasgun, the Lascannon is a flashlight that may be considered badass. Lascannons go FREEEM! or FOOSH! There is an ongoing debate on the pronunciation of "lascannon." As lasers are LAY-ZERZ, some players know the lascannon as the lays-cannon. YOU WISH IT COULD SHOOT POTATO CHIPS! It can't be called a debate when the powers-that-be already stated that it is pronounced like laser but with the "er" removed, but nobody at GW seems to actually stand by this ruling as several official videos, interviews, and audiobooks pronounce it the other way.
In all seriousness, a laser causes what it touches to explosively flash into plasma if the laser is strong enough. A normal lasgun is strong enough to blow an unarmored human nearly in half and, due to the way lasers work, can damage pretty much anything eventually. So, a lascannon does the same with far greater effectiveness. The lascannon beam doesn’t necessarily need to penetrate its target either. In many cases, the explosive damage caused by the hit itself will be enough to critically damage a target. The lascannon is, like the lasgun, a laser and does not "cut" or "bore" into a tank so much as make part of the vehicle explode with the force of a bomb. This thing will fuck your shit up and fuck it hard. Think miniature pyroclastic explosion. At your feet or your cover.
The lascannon is often placed as a phallic symbol on many Imperial Guard Leman Russes, because they thought the battle cannon was not enough. Spess Mahreens often use them in their Devastator Squads, because they are simply better than their Guardsmen counterparts, and want to make sure that's clear. But like their Guard counterparts, they like to stick them on all the tanks they have, because they are Spess Mahreens, and they're cool like that. Being an energy weapon, the Lascannon shares many drawbacks of similar weapons fielded by the Imperials in that it's not terribly energy-efficient, but it's hard to find a better anti-tank weapon or tool for reliably hurting lone high-threat units on the battlefield. Seeing as it is Strength 9, AP2, and Heavy 1, it really has no other use, save for what is ultimately sniping, only that vehicles and monstrous creatures are also on the list of targets, while squadded commanders are not.
Lascannons use single-charge power packs similar to las-gun power packs, but the size of a car battery and good for one shot. This means they have to be reloaded after each shot, like a missile launcher. Tanks with lascannons use a bank of these packs to help distribute the charge and recharge spent packs between volleys. They are most visible in the Predator tank and Sentinel walker. Theoretically, a Space Marine could hook up a scaled-up hellgun to his miniature nuclear reactor backpack and just let loose with some sort of hellcannon, but that would mean thinking and thinking is heresy.
The Lascannon is primarily designed for anti-vehicle combat, but it also sees use against monstrous creatures and the errant battlefield target that a commander needs to remove right the fuck now, such as isolated enemy characters. While it's not by any means the most powerful anti-armour weapon on the tabletop, the lascannon is still quite effective, and in the case of Imperial armies, it is available in quantity. Just bear in mind that when forced to operate outside its preferred role of burning down tough, high-priority targets, the lascannon is far less efficient.
If the Lasgun hits with the power of over double that of an AK47, then a Lascannon would most likely hit with the force of a 120mm APFSDS round, which by WH40K standards is sufficient to threaten nearly any armored target in the game, even super heavies like Baneblades and Titans if you get enough of them together. This is just a “what if 40k was equivalent to modern weapons”, we know that’s not the case just use your imagination.
It's hard to go wrong with the Lascannon. Ask for it by name.