o Communicating through a Firewall When building a distributed application with 100s/1000s of users spread over multiple locations, there is always the problem of communicating between client and server because of firewalls and proxy servers. Exposing your middle tier components as Web Services and invoking the directly from a Windows UI is a very valid option.
o Application Integration When integrating applications written in various languages and running on disparate systems. Or even applications running on the same platform that have been written by separate vendors.
o Business-to-Business Integration This is an enabler for B2B intergtation which allows one to expose vital business processes to authorized supplier and customers. An example would be exposing electronic ordering and invoicing, allowing customers to send you purchase orders and suppliers to send you invoices electronically.
o Software Reuse This takes place at multiple levels. Code Reuse at the Source code level or binary componet-based resuse. The limiting factor here is that you can reuse the code but not the data behind it. Webservice overcome this limitation. A scenario could be when you are building an app that aggregates the functionality of serveral other Applicatons. Each of these functions could be performed by individual apps, but there is value in perhaps combining the the multiple apps to present a unifiend view in a Portal or Intranet.
o When not to use Web Services: Single machine Applicatons When the apps are running on the same machine and need to communicate with each other use a native API. You also have the options of using component technologies such as COM or .NET Componets as there is very little overhead.
o Homogeneous Applications on a LAN If you have Win32 or Winforms apps that want to communicate to their server counterpart. It is much more efficient to use DCOM in the case of Win32 apps and .NET Remoting in the case of .NET Apps.