Why do we feel compelled to scratch bug bites if it's bad to do?

Scratching can remove insects from your skin while they are biting, or even before they bite, which could be useful. Scratching can remove irritants of various sorts, although we also sometimes scratch when there is no irritant that we can remove. Itching is caused by various things, and the scratching response is sometimes useful and sometimes useless or actually harmful to some degree. Our biological design is not 100% efficient.