More contact surface. More grip.
Rain, or dry roads and sunshine. What's more reliable than any weather forecast is: the wider your tyres, the more grip they'll give you - apart from if you're aquaplaning.
What floats better: An elephant on a small rubber dinghy, or an elephant on a big one? Precisely. Wide tyres have to displace more water and are quicker to float when aquaplaning.
Minus point: comfort.
Due to the lower tyre sidewall, wider tyres are considerably more rigid, making them easier to control, yet less comfortable. You'll be a little louder on the asphalt.
More resistance. More consumption.
With wider tyres, a larger surface is exposed to the wind - 6% on 30 mm, more, to be precise. That means: Rolling resistance increases. Your Golf VI needs more power and uses more fuel as a result.
More centimetres for more hp.
A rule of thumb is: The wider your tyres, the greater their performance on the road; which is why the tyres on the Golf VI are 5 cm wider than those on the Golf I. Ultimately, it has a greater hp.