Discussions and comments
A ticket almost always raises several topics
Attachments and images
A mockup, a specification, a screenshot of the bug: much of what belongs to a ticket is not a line of text. Attachments keep such files right at the ticket instead of in a chat or on a drive nobody finds later. An image can also be placed on the card itself as a card image.
Involving stakeholders
This feature is being rolled out gradually. If you don't see it yet, it hasn't been enabled for your workspace.