HTTP Interceptor

By extending the HttpInterceptor class you can create a custom interceptor to modify http requests before they get sent to the server. Http interceptors are added to the request pipeline in the providers section of the app.module.ts file.

 

for more : 

https://angular.io/api/common/http/HttpInterceptor 

You may use it for adding headers to all communication or check for them.

Basically they are injected to all requests / responses to change / inspect them.

I use it often for adding authentication token to all the communication to be evaluated on serverside.