Help! writing UCAS personal statement and need reasons of why study economics!

You can describe your reasons for wanting to take up a career in finance, and connect that to your choice of major. If this is a graduate statement, you'll REALLY need to flesh it out, but definitely focus on your goals--what you'll do when you graduate. If you're undergraduate, focus on why you'd excel; in the subject and why you need it for your career. Outlining the statement's contents would be a good idea at this point, as would really beginning to narrow down your choice of occupation and doing background research. Statements are usually soul-searching, labor-intensive and in the long run they are so worth it, as they show the University what you want from their school, and they'll show you too. But! Whether you are a graduate or undergraduate, do NOT sound as if you haven't decided or that you are unsure of yourself, even if that's the case.