This is common practice, but it’s playing into a bad power dynamic where the employer is trying to anchor the negotiation to a number. It’s a predatory question, and giving a bullshit answer to a bullshit question is a reasonable response.

As I mentioned in another comment, I personally prefer just setting a salary range I’d be willing to accept. I walk away if given an offer below that range. It gives me the initiative of anchoring the negotiation to a number I want.

It also saves me a lot of time by screening out companies that aren’t willing to actually negotiate.

So why not just tell them a number, because the company is going to go for the lowest in the range anyways?

You can ALWAYS negotiate before you accept the offer.

"Yeah thanks, but that offer is not enough for me to leave my current job. I need at least X more. Let me know if you're interested."

(In fact, you can negotiate even AFTER you accept it!)