Besides the built-in property pane field types, you might have a need for custom field types within the property pane. Usually, you should suffice quite well with the default field types, but at times you might find yourself in a situation that is best resolved with a custom field type. These are primarily used to render custom HTML.
Building a custom field type is not trivial, so consider for a moment before building your own. This tends to complicate your code and might bring future issues if or when Microsoft should update the collection of available field types. These changes might, in turn, render your custom properties redundant, and force you to clean up code for custom web parts.