Because if the people don't want to join in the first place, why hold negotiations to begin with?

Parliament does not really have a majority vote for continuing talks. But one of the government parties made it one of their election issues so they want to find out if there is a majority support among voters.

When people negotiate, the terms of the deal are affected by many factors. Imagine Greenland "cashes in" IOUs/goodwill to get better terms, only for the referendum to fail. Then they change their mind a couple years later, maybe because an ally threatens them with invasion - they won't get the same deal again.

Making requests to humans is never idempotent.

Because you’re in a different position to negotiate if you know you have the support of the population? That’s my intuition at least

You have to apply to the EU to start negotiating

I guess one does not “just negotiate an agreement” for something like this.

And perhaps two referendums and working incrementally is wise in general, given the mess that was Brexit.

What agreement? To join the EU you must change a lot of internal structures in your own government first, a lor of regulations must be changed to align with EU versions, laws have to be changed, even basics, like drivers licences and other documents.

Why spend millions if not billions on all that, impose laws that are maybe bad for normal people, and then figure out that people don't actually want to join the EU?