The problem with HTTP alone is that it is mainly a mechanism for passing files from server to client. Create an extensible protocol and payload format that can evolve over time. The authors of the specification explicitly exclude the more involved aspects of building an object model, as well as anything that's already been built. You would need to build them yourself using whatever tools you have available. SOAP, defined SOAP defines a mechanism to pass commands and parameters between HTTP clients and servers.
SOAP is simple.
The client sends a request to a server to invoke an object, and the server sends back the results. The messages themselves are formatted in XML, and encapsulated in HTTP protocol messages. SOAP works with existing Internet infrastructure. You don't need to make any special accommodations on any of your routers, firewalls, or proxy servers for SOAP to work. In this case, the program knows this is the way to format a request for a stock price.