If you ride more than a couple of miles a day, it's worth investing in the proper clothing. Riding in regular clothes can be uncomfortable, sweaty and potentially dangerous. Cycling clothing is generally tight-fitting for comfort and warmth, with the aim of reducing wind-resistance – a baggy jacket acts like a braking parachute and also lets cold air get closer to the skin.
Apart from making the rider more streamlined, the other main function of tighter clothing is to keep your skin dry. It takes sweat to the outside where it can evaporate without cooling the body too much and without soaking the clothing.