The ESP (electronic stabilisation program) on the Mk V GTI is the third generation ESp on the Golf and is designed specifically for the GTi. It will allow more wheelspin and slide than previous versions or indeed ESP on any other Mk V.
As previously mentioned, pushing the button turns the system OFF. Reasons for wanting to turn it off vary but generally you would want to turn it off in extreme conditions. eg. You were stuck up to your wheelnuts in mud. You try and pull away and nothing will happen because ESP detects no traction and retards engine power/brakes the wheels. Another example (and something that happened to me and cost me two wheel refurbs on a MK V GTi). Last winter I drove into work after a night of snow/ice. I left ESP on as was driving on crappy but gritted roads. I turned left into a side road which hadn't been gritted and evidently was like sheet ice. Straight away I obviously slid (no more than 20 -25mph) gently towards the kerb and tried to put my foot down but of course nothing happened and I looked like a complete muppet as I gently bounced into the pavement. If ESp wasn't on, I'd of been able to hopefully put some power down and regained grip without kissing the kerb.
Also, ESP3 as on the Mk V GTi won't retard engine power as on other versions of ESP when flooring it out of junctions for example. It will allow reasonable wheelspin upto 30mph.
