The stated purpose was to flatten the curve.

> Who benefited?

The government.

> How?

By giving the impression that they were doing something.

> Why were the curfews necessary to achieve those goals?

They weren't, as far as we know. In Quebec's case they were still scrambling to justify them a few hours before the press conference:

https://www.thesuburban.com/news/legault-s-curfew-decision-w...

https://www.cbc.ca/news/canada/montreal/curfew-legality-queb...