I would never have transfered my task management to todoist if it was only a trial. It's a fair amount of effort to switch over a large chunks of how you function. I'm not going through that effort only to find out the product might be okay, but not enough to justify a subscription, at least not initially: Starting off often means you grow into using it more, more often, more features. A 30 day trial doesn't leave enough time for that.
Then there's word of mouth: A free plan I use regularly will be much more top of mind when talking to others.
Also bootstrapping: It's all well and good for an established SaaS business to not want a free plan, but plenty of products might never get to that point if they didn't bootstrap with a free plan first.