Excerpt is short extract from a post. If you want to display excerpt on your wordpress website page.
Although wordpress user profile has pretty much every thing inside it. But in case if you want to add your own field to user profile pages.
Revisions are stored as separate posts, the more revisions the more space they will take inside database. To avoid this database overhead there is a constant inside config file for maximum revisions.
CDN Content Delivery network is the most reliable way to getting content from internet, then why not add jQuery from CDN.