Servlets can be thought of a 'plug ins' to a web browser. Bits of logic you write that take a web request, do a bit of work and return a new page (usually via JSP).
It is totally possible to write your logic as a series of Servlets and then have a stand along front end (using an HttpURLConnection as stated) to connect to the web server, get the bit you want and parse the output (HTML or even XML).
The more common approach is to deploy the logic as a series of EJBs. Both the Servlet and Stand Alone App would be clients of the
EJB. This allows you to code and deploy the logic only once while making use of it in several locations.