Because You Don't Live in a Suit
Sport Coats are a great option for a gentleman that doesn't need to dress in a suit everyday. Don't confuse a sport coat with a blazer however. While worn in a similar fashion, a blazer will have a metallic button and will likely be a solid navy or black wool or cashmere.
A sport coat can appear to look like a suit coat in some cases but does not have a matching trouser. The sport coat and blazer add versatility to an assembled wardrobe that allows the wearer to adjust comfortably to his surrounding by adding or shedding a jacket during the day. Many options are available in lighter weight jacketing for summer and likewise heavier jackets for winter and fall.
Look for cottons, seer suckers and silk/wool blends to stay cool in the summer and worsted wool, flannels and tweeds to keep you comfortable in the wintertime.
In today's casual environments, the "jacket and jeans" look has become increasingly popular.