The Windows Process Activation Service
Windows Process Activation Service generalizes the Internet Information Services (IIS) process model, removing the dependency on HTTP. All the features of IIS that were previously available only to HTTP applications are now available to applications hosting Windows Communication Foundation (WCF) services, by using non-HTTP protocols. IIS 7.0 also uses Windows Process Activation Service for message-based activation over HTTP.
Conditions for installing and running WAS
From a security standpoint, you should not run Windows Process Activation Service unless you are supporting a .NET Framework 3.5.1 application or IIS 7.0.
IIS and .NET Framework 3.5.1 automatically install Windows Process Activation Service as needed. You should not install Windows Process Activation Service by itself.