There are various reasons as to why a tooth may need to be extracted.
Orthodontic Extractions are elective extractions requested by an Orthodontist, usually these are completed just before fitting of fixed appliances (braces).
Broken-down/infected teeth sometimes are unable to be treated with conventional root canal therapy or patients simply elect to have the tooth removed. Removing a severely broken- down/infected tooth will be beneficial to improving oral health and indeed whole body health if the infection has been present for some time. There are many replacement options to restore function & aesthetics ranging from implants to bridges or partial dentures. (link)