So being fairly new to the game and playing almost exclusivly Imperial, perhaps it's my noobishness showing through here but can some one explain to me why HLC is such a big deal in most metas?

I realize it has the potential to hit more but ive had mixed sucess. I'd much rather take an Auto Corrector with a Mangler or even just the Mangler. It's cheaper and on average seems to hit just about as hard.
As far as rules go what exactly is the synergy of this particular cannon that makes it worth 7 points.

Against something that's easy to hit, the HLC just adds an extra die's worth of damage--which is helpful but not crucial. But against something that's very hard to hit, it can often make the difference between hitting and not hitting. Especially against Stealth Device, that's critically important.

A couple of reasons.

A) Secondary weapons do not receive a range bonus at range 1, nor does the defender roll an extra evade at range 3. This lets HLC hit from afar while in a defensive position (receiving the +1 green on rolls against you)

B) 4 dice is more than just 33% more than 3 dice if that makes any sense. When fighting high agility targets like interceptors, the likelihood of dodging 3 hits is not all that difficult, 3 green + an evade token (or autothrusters). The likelihood of evading 4 is near impossible. And when a ship only has 3 hp, that makes all the difference some times. That's not to say you'll roll 4 hits, but since red dice are 4/8 hits vs green's 3/8, you are more likely to roll 4 hits than 4 evades. With focus+TL, the odds of 4 hits is actually pretty easy, 77%. Whereas the odds of 4 evades with 3 dice with focus+evade, is a pitiful 24%. With 3 dice, the chance for 3 evades with focus+evade is significantly better at 39%.

But this isn't a game of statistical averages. One bad green roll in some cases is all it takes. And it is far easier to roll a terrible green throw than red (odds, and TL). Red dice just always beat green dice.

Its 4 attack dice, which is good against even the most agile ships, and unlike a primary weapon attack you don't grant the defended an extra defence die at range 3.

On its own, turning crits rolled into hits is the tradeoff for powerful long range shots - but combine with certain pilot abilities and EPTs and crew abilities and that is where it shines.

My personal favourite being Rexler Breath - 4 red dice at range 3, spending a focus to turn any damage cards dealt faceup after they are dealt.

Mangler is a very nice mid cost cannon, but lacks the strength to beat anything with high agility (3 or 4 green dice) consistently.

It is the penetrating power of that extra dice that makes the HLC worth the points. Cloaked phantoms and hopped up Interceptors can shrug off three crits as easily as shrugging off three hits. With 3 agility (of four if using stealth), and an evade, it is difficult to land even a single hit with only three dice. That extra die makes it easier to land a hit on a high agility opponent.

Plus all the good reasons listed above.

The Mangler is still semi-new. For quite some time the only choices for a cannon were HLC, Ion or Autoblaster, of the 3 the HLC is clearly the better choice.