Why isn't the crown finished right away instead of going through a temporary tooth?

A temporary tooth is used partly as an intermediate step to raise the quality of the final restoration, and partly to protect the tooth that has been trimmed. During the temporary period we check 1) whether the chewing height (occlusion) is comfortable, 2) whether the gum heals without irritation, and 3) whether there is any problem with speech or appearance, and we reflect the findings in the final restoration. It also protects the prepared tooth from sensitivity and keeps the teeth from moving. It is usually worn for about 1 to 2 weeks, and longer depending on root canal treatment or the state of the gums.