This is primarily down to the fact that each candidate is competing for a paid role with a graduate salary. This alone makes it vital to ensure that each candidate is questioned and put on the spot about their policies, ideas and intentions in the role. Otherwise, candidates would gain a valuable paid job for what is essentially a popularity contest.
One could make the argument against the debates that they aren’t very effective, and are often seen as being run unfairly in favour of certain candidates. However, part of the issue with the debates is that there just isn’t enough interest or exposure. They would be far more effective with a bigger audience that care more about their representation and thus the results of the SU elections.
The debates are currently something of a formality. Much like persistent flyering and covering the students' union in posters, it’s something all the candidates do, but arguments about how effective they are can be made for both sides. The debates allow people to really get to know candidates, and crucially put pressure on them to defend what they believe. However, the impact that the debates have are so minimal compared to almost every other part of the campaign that it must be questioned how necessary they are in the grand scheme of the elections.
The debates need more preparation, exposure and interest
With more preparation, more exposure and more interest developed in the debates, they would be a far more effective tool. They would provide a platform for candidates, as well as an opportunity for students to have their questions presented and answered by those who will take hold of the University they attend. They would also help hold candidates to account, something desperately needed.