"Uttering Threats" in Canada is just their description of an Assault charge, "uttering" meaning "giving", like how "real property" does not mean "physically existing" but refers to fixed property and attached rights. You would be convicted of uttering threats if you wrote on a chalk board that you were going to murder someone.
Looking at it it's likely a hold over from British law, we updated the statute's name to "language likely to cause a breach of the peace" and there is another one that covers the same thing but in a different set of circumstances.
For example - If I said to a friend "Mate your a lucky fucking sod I really want to kill you for getting that deal" that wouldn't be constituted as a threat under most circumstances, but if I said "I'm going to kill you you fucking cunt" to a random stranger or someone I have just had a negative interaction with that would be considered a breach of the peace and a arrestable (though not often charged) offence.
Technically if a police officer heard me say that to my friend I could hypothetically be arrested, but the Desk Sergeant and the CPS would toss it out unless said friend came to them and said no he's a danger to me and meant it meaning that it would go not where and the most I'd get would be "Please chose your language better in future" and the officer would get a talking to about stat padding, but if the same officer heard me say that after getting booted out of a pub by a doorman and I was arrested on suspicion they could charge me with it and it would likely stick especially if I was in possession of something that could be used as a weapon and I'd catch a charge for that as well and the other charge would be used to compound the charge.
It's one of those laws that's useful to have hanging around for the Police and Courts, it let's them add a little more "oomph" to other charges, and it can be used at protests and riots as well to quickly and legally deal with agitators before the bricks start flying, without having to resort to the Riot Act and it's shockingly effective. It can be abused and has been abused but it's a "At descension" sort of law so if I was getting mouth with a Policeman I could be charged with it and the bracelets slapped on me but it's highly unlikely it would go more than being stuck in the back of a police car / van till I calmed down and given a verbal caution.
It's a silly law for the most part but it can be useful to keep around, it's why when they update rules relating to traffic and road use, they keep the inclusion of Horses, Carrages or a Horse, Bicycles, Locomotives, skateboards, roller skates and all other non pedestrian means of transport, it's so they can arrest the guy who'd drunk trying to ride a bike he's more of a danger to himself than others and it's normally a case of charge him with that slap the cuffs on him issue a FPN and take them home to sleep it off. But the same law can be used to stop illegal pony races at 1am in rotherham when the pikeys get drunk and decide to use a dual carriageway as a impromptu racetrack.