It's a misconception that ASP.NET Web API has replaced WCF. It's another way of building non-SOAP based services, for example, plain XML or JSON string etc.
Yes, it has some added advantages like utilizing full features of HTTP and reaching more clients such as mobile devices etc.
But WCF is still a good choice for following scenarios:
If we intended to use transport other than HTTP e.g. TCP, UDP or Named Pipes.
Messag Queuing scenario using MSMQ.
One-way communication or Duplex communication
A good understanding for WCF(Windows Communication Foundation), please follow WCF Tutorial.