Commuting is substituting the bicycle for the car, but as opposed to the driving a car it's a physical activity, and that for most of us means excessive sweating.
Even if I was moving at 5mph I would arrive at work sweaty since NYC is usually humid and I don't take humidity too well. My ride is around 8 miles (~13km), there are people whose commutes are even longer than mine too. There are less than 10 days per year here when the weather is cool and dry enough so I don't sweat enough to have to clean up at work. Besides going slow isn't fun most of the time. European countries have drier climate and shorter commute distances. If my ride was 3km in a dry climate I could probably wear a suit an wouldn't break much sweat.
Also, the weather in NYC is pretty unpredictable. It can be nice, warm and dry in the morning and chilly and rainy by the end of the day, or vice versa. So since I like certain amount of comfort and I like to be clean, dry and fresh at work I need to carry some extra clothing in addition to a daily change of clothes (this adds more weight in cold months, of course). I also carry some tools since bike shops don't open until 10am so I want to be able to do most common repairs. Plus lights, batteries, some gadgets like two smartphones, small portable hard drive, etc. It quickly adds up to around 20lbs or more and I don't even carry a laptop most of the time, sometimes a netbook or an iPad.
Every now and then I need to re-supply my toiletries, I bring them from home because buying stuff in Manhattan is ridiculously expensive, I do my shopping at Costco and BJ's in Queens. I bring a fresh towel once a week. Rarely I bring lunch along with me but I usually have some snack bars scattered across the bags.
I sometimes stop by street vendors and pick up some fruit. Sometimes I do some gigs after work so I carry a laptop, some computer parts and tools. In such situations I probably hit 60lbs easily. That's why my commuter has 4 panniers. People often ask me if I'm on a tour
Before I started to commute by bike regularly I used to carry a huge a heavy backpack, very uncomfortable. Now, it's so much easier to carry stuff on a bike, I don't even feel another 10lbs.