Roads get slippery for a number of reasons. Rain and snow are obvious ones, but sometimes oil and other automotive fluids drip out all over the road. Steer your bike around any weird puddles you see on the road if you can, but sometimes stuff just coats the surface of the road and makes it slippery and there's no way to avoid it.
When you put your feet down at a stop, you want to be sure that whatever shoes you're wearing will grip the road securely, not slip and slide and make you feel like you're going to drop the bike. That's another reason why we recommend motorcycle-specific boots, but any good non-slip shoe is better than a slippery-bottomed (but sick-looking) sneaker. Before you get mad, keep in mind that not all shoes have equal grip on all surfaces. Some shoes might be perfect for a polished wood gym floor, but suck at slick asphalt. Shoe makers make different sneakers to suit different athletic pursuits, so this isn't exactly a surprise.