I'm sure this is true in a lot of professions, but I'm in law school and I can assure you that a suit is often non-negotiable attire with the more traditional law firms. The firm I clerk for is more casual, but when my boss has an out-of-office meeting, he wears a suit.

A suit is also standard attire for court. Some judges explicitly state in their rules they require a suit (and often require a skirt or dress for women), while others rely more on custom and word of mouth to let it be known a suit is required.

If I were to show up to my Moot Court semi-finals next week not appropriately dressed in a skirt suit, pantyhose and dress shoes, I wouldn't be allowed to argue.