Is there a point to downvote a question from a user that has the minimum reputation score available and which question has already a negative score?
Yes, the same reason you'd downvote any question, for the downvote reason:
This question does not show any research effort; it is unclear or not useful
Downvoting questions lets the OP and future visitors know that questions like that are not a good fit for the site in their current state.
I see downvoting questions as a way to show my opinion of the question. I don't think of it as a way to punish the user by taking away reputation. It doesn't matter if they have a lot of reputation or 1 reputation, the downvote is for the question, not the user.