When applying for a job, why do they ask you to upload your resume, then turn around and recreate your resume on the job posting site?

1) to see how well you follow directions

2) to see if you are consistent, or if you are a liar

3) they’re stupid

Making it more scannable and searchable. They’re looking for keywords, probably using a software routine. Make sure you know what they are, and use them. You may get no more than a fast look as a first cut, so you have to get sorted in, not out.

How do you present yourself in your whole, intact resume? and how well do your qualifications fit a profile of an opening? Those are two different things. For example, being able to fill in a certain field gives no indication of whether you can write a coherent sentence. In some jobs, that actually matters.

Maybe also looking for possible fit with other openings if you don’t score a hit on the one you’re responding to.

Also shedding special fonts, weird characters, and other typographic and formatting tricks that applicants may use (the way they used to use deckle-edged creamy rag bond) to try to get attention.

And, of course, better to waste your time than theirs. Why should they devote resources to considering you if you can’t be bothered to comply with their requirements?

Because HR is lazy. Some of their systems hit on keywords and filter applicants based on this. They don’t know what you do, how your previous experience applies or really what the job you are applying for really is. They’re matching what you write on your resume to the job description. They don’t have the time to read 100’s of resumes so they fall back on their keyword systems.

Can’t they scan for keywords on my Word resume?

It’s easier for them to use a system like this. It’s also a filter for those who are not really serious.

As @Jeruba mentioned “They’re looking for keywords, probably using a software routine. Make sure you know what they are, and use them.”
I’ve heard it more explicitly suggested to cut and paste key words and phrases directly from the job listing into your resume.

I understand that doesn’t solve the nuisance online forms issue, I agree with @ARE_you_kidding_me that is probably because “HR is lazy”.

That’s what I’ve heard too. I’ve known about that since they started using key word searches in the 90s.

Musta done it right. Had a phone interview yesterday.

At one point we were talking about MS Office programs. I said Excel was my favorite program.
She asked “What is your favorite formula.”
Wait…what?
I said, “The one that allows me to link to other work pages or work books, or to Word files or to the internet.”