Serious question: Why is asking your salary any less relevant then asking for your previous place of employment? If I say I did X on project Y at Google, that's a pretty incomplete picture of how valuable my coworkers and bosses found my work on project Y. Thus, it seems like a salary is another potentially helpful data point indicating how valuable I've been to previous companies.

Obviously in a better world it would be possible to glean this information from an interview alone, but the bottom line is that interviews generally suck at informing how useful a candidate is going to be. A candidate who claims they've done valuable work is potentially exaggerating, and it's difficult to tell by how much. That's (part of) why we have resumes, and (most of) why an employer wants to know your previous salary.

Note that I'm asking this unironically. I know that "never tell anyone your salary" is a popular point of discussion here, but I have trouble seeing why it's any different from, say, a grad school asking for your GPA.