If you're doing maths A-level, you've probably already decided there's at least one good answer to that question, but you may be trying to decide what to do next, so a few more reasons might help.
All the reasons given in this page, from Leeds University, are good ones. Mathematics does help you understand the world - especially a world in which figures are constantly thrown around, often it seems with a deliberate intention to confuse. And maths is invaluable not just for a career in mathematics itself but in pretty much all science and engineering disciplines and a lot of the business world as well. So by doing maths at university, you'd be keeping lots of options open. You could even go into politics one day and became that rarity, an MP who actually understands things like probability and exponential growth.
I once bumped into someone I had worked with long before, soon after we both graduated. By the time we met again he was a professor of engineering at a UK university, and we had both had multiple jobs along the way. He said to me, "Every time I leave a job, my main regret is that I didn't make the time to learn more maths". That's true of me as well, so you may as well get ahead of the curve with some tutoring.
In case you're wondering about Leeds in particular - the university is pretty good, and the Yorkshire countryside is wonderful. If you're a football fan, though, you might want to think twice.