It is possible to include profile properties in an event. These properties are characteristic of the user. These properties are persisted across events, whereas the event properties are only available and relevant to the event itself. Profile Properties allow you to store information on a user that you might need later. For example, the Converge Pixel will auto-track a bunch of browser session information, so this is available for your server-side events (e.g. the Placed Order event in the example above). However, profile properties are also useful for storing information across events within a single source. Imagine a user that triggers a Subscribed to Newsletter event with an $email parameter. Afterward, that same user triggers an Added To Cart. Storing the $email as a profile property ensures that you have this information available at the moment of the Added To Cart event without you as a developer having to pass this information again.