A lot of traffic to sites (in general) comes through search engines and generally land on a subpage of a site. Not a problem, but say that I’d want to welcome that type of visitor with a message (not an alert mind you, just some inline text) telling them they can find way more if they start out on the main page.
At the same time, visitors who are just browsing through the site and stumble on such a page should not see that message.
My Agent stack allows you to customize the content of a page based on several different parameters. Parsing a referring URL is not reliable. However, if you have a link that passes URL parameters, you can easily use Agent to detect when that parameter is passed and then display certain content.
You may also want to look into my Call to Action stack that allows you to create business rules for when a person can receive a CTA.