One is friction on the other side. With Doodle you're asking someone to click a link, open a UI, parse a grid of times, check boxes, and sometimes connect an account. That's a real ask, especially for someone external who has no relationship with the tool. With Vela they just reply "Tuesday works" in the thread they are already in.
But beyond reducing friction, Vela is also doing the actual coordination work: herding people, following up with non responders, suggesting specific times that work best (not just available ones), handling rescheduling, and closing the loop. It's closer to what a human coordinator does than what a poll does.
Our customers are mostly folks coordinating 30+ meetings a week across multiple people. For them, time spent compounds significantly.
Doodle is great too btw, but it really only works well when the people involved already know each other and at a small scale. Vela is built for the more complicated scenarios where companies have tried everything and decided nothing works but putting a team member on the job.