Yes, it's a negotiation and the negotiation is bounded on one side by 'what the job/employee is worth to the employer' and the other side by 'what the employee is willing to accept.' My point is that your previous salary strongly dictates the latter and thus heavily affects the negotiation.

And if you don't like the 'fishing for a raise' terminology we can instead just say that it signals that you don't think your previous salary is a strong point in your negotiation. So now the prospective employer doesn't know your salary, but suspects that the negotiation window is quite wide.