AS Levels prepare you for the A2 the following year.
It's a big step-up in terms of coursework and exam preparation from GCSE's.
Also, when writing up your CV (basically a sheet of paper that lists your qualifications, work experience, hobbies etc that you give to employers for jobs), it's beneficial to add your AS results (either predicted or confirmed grades) in addition to your GCSE's.
I remember doing my AS Levels. I should have left my 6th form centre to study them elsewhere, but 16/17yr old me wanted to stay with my friends. Sadly that meant I was studying courses I had zero interest in, and in the end, I did fail some.
Got pushed back a year and had to re-sit them in a different college.
Edit: 'and pay the up-charge price of £9K a year. (If I had moved straight away, I would have paid £3K a year)' - This is in relation to university fees, as I did attend after completing my A Levels. I just missed out big time on the cheaper price for uni.