Tags are a great way to further organize your transactions. Tags are much more flexible to use than categories as there aren't so many other items associated and dependent on a transaction's tag.
A transaction can have any number of tags, from 0 to as many as possible before it looks wonky in the UI.
You can set the color scheme for any tag from the Tags page.
You can generate a report or charts based on tags from the Analyze page.
You can break down your spending by tag, or filter transactions by tag from the Transactions page.