I would assume it's the first thing they read after your name.
I would also assume they get flooded with a lot of applications and that most of them are from people who are "grossly under qualified".
Your challenge is to write a couple of sentences that suggest you are good at what you do and that you can benefit their team.
Remember, the person reading this probably has another 50 or 100 to go through and is looking for an excuse to hit the 'delete' button.
You might want to also include a cover letter if it includes more narrative than fits in the introduction. Just be darned sure you've written a custom letter for this application and that you make specific references to their company, the position, and how you'd be a fit. No one likes to read "one size fits all" cover letters.