What is the purpose behind an Ofsted inspection?

What is the purpose of an Ofsted inspection ; to help a school improve or catch them out?
This question arises out of a couple of things that have come my way recently: one was Mansell's article in TES about Ofsted being more interested in its image than anything else (Really? Well knock me down with a feather!). The other was a recent LA inspection by Ofsted trained inspectors at Mrs Admin's school. The LA was to provide a 'pre-Ofsted' inspection for the school under Ofsted conditions and using Ofsted registered inspectors; the idea being to help staff prepare for the next real inspection, which is due any time in the near future. Ok, well, good intention - and anyway, you can never have too many inspections, can you?
Well, the chap inspecting Mrs Admin's department made his intentions clear as soon as he walked in: "Now, no need to worry. I'm not here to catch you out, I'm here to help you improve and look at all the good things you're doing in this department". And he was, indeed, as good as his word. Some excellent lessons were observed and very positive feedback given. Points for improvement were delivered in positive and encouraging language, all the time referring back to what was good in the department. Result? One motivated and happy set of teachers, pleased that their efforts had been appreciated. That department is now up for the challenge.
Sadly, the story was not repeated in other departments seen by different inspectors. The general feeling was that the inspectors were looking to catch the teachers out. Feedback tended to concentrate on the negative. Result? A demoralised and unhappy set of teachers with an increased cynicism regarding both the value and purpose of inspection.
It's my feeling, and indeed experience in the classroom, that too many Ofsted inspectors fall into the latter category and that often a decision is made even before the on-site inspection, based on the exam results (no, don't get me started!).
I was at Mrs Admin's school last weekend for the Christmas fair. It was packed. Huge parental support, lots of kids there helping out and, best of all, Christmas music provided by the junior band - then the senior band - then the school orchestra - then the swing band! And where was Ofsted when all that was going on, eh? There's more to a school than exam results and four-part lessons.