Canadian political parties, when in power (and it's not a partisan thing, they both do it) see fit to "advertise" their party under a thin veil of pretending to advertise government programs. The example that immediately comes to mind (despite my distain for the current government) is the former government who circa 2008 had advertising everywhere for their "economic action plan" and how it was helping us. It was partisan garbage paid for by taxpayers. They all do it.

This is my take. Bad law, but the government should never have been advertising there in the first place.

Same reason it, and other governments advertise programs, PSAs, etc, on television, YouTube, etc. It reaches eyeballs.

We have tax-backed CBC for that (+other tax supported media)

Which doesn't reach people who don't watch CBC/listen to CBC radio. Many young people don't do either.

Most government advertising is brand/awareness advertising, which is pretty cheap when other brands aren't competing for the same keywords as you.

And the cost of running a government ad on CBC is non-zero. It could be occupying space used for a program people actually want to watch/listen to.

Tourism.