Link fields provide an easy interface for content creators to provide links to URLs and email addresses. For external links, content creators choose "external". For email links "email address". Finally for content hosted on the HubSpot CMS they can use "content" which shows a list of all pages and blog posts in the account, file showing file assets, and blog, showing all of the blog listings in the account. Link fields are very similar to URL fields except for the key difference that they provide a UI for "open in new window" and "tell search engines not to follow". If you do not want content creators to have that control use the URL field.