Why does the HR ask if I'm on permanent payroll or on contract?

If you’re looking for a contractor position, on the other hand, they’ll refer your application to one of the staffing agencies which provides them workers with skillsets similar to yours.

First of all, unless you are interviewing for a position in Human Resources, HR won’t interview you. Too many rejects have been let down easy with the statement “we’re waiting for HR’s approval” or something like it. It’s the manager of the department with the vacancy who conducts the interview.

What I glean from your question doesn’t sound so much as an interview, but more like information entered into an application. As it is, employers want to know which type of employment you’re looking for. If you’re looking for a permanent position, they’ll process you within the company, interview you, and possibly hire you directly.

If you’re looking for a contractor position, on the other hand, they’ll refer your application to one of the staffing agencies which provides them workers with skillsets similar to yours. The staffing agency will most likely ask for your résumé, have you fill out their application form, and interview you before they forward your résumé and send you to interview with their client.