TV debates provide a way to see politicians interact and challenge each other's policies directly, how they carry themselves and gauge how well they can think.
Much of the other forms of campaigning are uni-directional and not necessarily all that challenging. When candidate X says on the campaign trail that he'd "deport all immigrants", that does not give much ground to challenge whether it would be a good policy. The crowd can generally be expected to be pro-X and in any case a rally is not a Q&A forum.
When candidate Y says on a network interview that she would "amnesty all immigrants", the same mechanism fires in a different way. The interviewer may be throwing softball, the format may be agreed upon in advance. Again, pitfalls in that policy may not be sufficiently thrown into light. And, by now most networks are watched by one side's supporters rather than the other.
Even later press critical coverage of a candidate policy statements lack both the immediacy and the need to respond: the candidate can ignore the questions entirely unless their campaign team sees a need to respond.
In a debate format - TV or not, but we're long past radio at this point - this comforting ease drops away: the candidates will have to convince the larger electorate - their side's, their opponent's, plus undecideds are watching - that their opponent's concerns about their policies are unfounded and that these are workable, no-nonsense, not-just-campaign promises. Their opponent has a vested interest in challenging them on every shaky bit, not giving them a pass. To go back to my example, past the soundbites and the spin, how solid are X and Y's policies with regards to immigrants? Are they workable? Costly? Principled? Geared towards their political "base" and not relevant to the average citizen?
You, the viewer, get to form your opinion largely without the - possible - filter of news coverage that favors one side over the other, in a non-scripted environment.
A debate may also allow viewers to gauge the charisma and capability to convince of a future leader. That matters in itself: like them or not, Reagan's policies got implemented because he was considered "The Great Communicator". That contrasts to - perhaps - better-intentioned and better briefed leaders that struggle to manage policy because they lack charisma.
And poor Nixon lost his first debate because he appeared shifty and untrustworthy. Which wasn't all that far off.
Granted, as you look at its website, Leaders’ Debates Commission, the following is far from neutral, but it still does a good job conveying the "pro" side for debates:
Debates play an essential role in Canada’s democracy. They give you a chance to see the character, temperament, and unscripted approaches of leaders seeking to be Canada’s Prime Minister.
Making an informed decision is part of a thriving democracy. But it’s difficult to make an informed decision; people are busy, and it’s hard to know what information to trust. A live debate is a trusted source of information, because it’s one of the very few times during an election campaign that you can hear directly from leaders – unedited and unfiltered.
You’ll see leaders together in one place, live on stage, answering questions and challenging each other’s ideas and opinions. This will help you make your own decision about the issues that matter to you. It is also an opportunity to learn about issues that matter to other Canadians, and be exposed to views that are different from your own, and those around you. It’s a window into the world of others.
Are they "essential"? Possibly not, but they are far from negligible and the reasons for refusing to participate in debates is a factor to consider when assessing a candidate.
p.s. The specific election system doesn't matter all that much - Canada uses a parliamentary system yet airs debates between party leaders, much as the US does with POTUS wannabees.
p.p.s. This importance I attribute to debates is an opinion and apparently not a universally held one. But it is a point of view shared by at least some experts in the field:
What factors explain the broadcasting of televised election debates? Empirical evidence from Germany
Televised election debates are considered as the most important single campaign event.
The Television Debates: A Revolution That Deserves a Future on JSTOR
p.p.p.s
All the same, one also has to remember that debating skill is not a foolproof indicator of good leadership by itself and expecting debates to magically "solve" democracy is unrealistic. Consider how different the world would be now if one of two participants in a certain set of debates in 2016 had been wittier, more likable and less wooden. Yet, a sharper tongue does not, by itself, a good leader make.
