Red Flag Question?

So I applied for an office admin position at a marketing firm. Spoke on the phone with them and set up an interview for later this week. Then she was like I’m going to send you an email that just has some questions we’d like you to fill out before the interview. I’m like ok fine easy enough. So I open the link, and the questions are ‘what was your previous position’ ‘how long did you work there’ and I’m thinking ok well…this is all stuff you should’ve been able to learn from my resume but ain’t that the way it goes these days.

And then it asked me what my salary was at my previous job. I found this…odd. Why would they need to know that? This is obviously the company taking a reading on how much they can underpay me, right?

EDIT: Upon further inspection, employers asking job applicants about their salary history is actually illegal in the state of Illinois, where I am. I think it would be best if I did not work for these people.