Why would you ever call .call() on Observable functions?

Question

I am a relative beginner in Angular, and I am struggling to understand some source I am reading from the ng-bootstrap project. The source code can be found here.

I

Answer 1

I suppose

const inputValues$ = _do.call(this._valueChanges, value => {
  this._userInput = value;
  if (this.editable) {
    this._onChange(value);
  }
}); 

is equivalent to

const inputValues$=this._valueChanges.do(value=>{
 this._userInput = value;
  if (this.editable) {
    this._onChange(value);
  }
})

In my opinion it's not an usual pattern(I think it is the same pattern but written in different fashion) for working with observable. _do() in the code is being used as standalone function take a callback as argument and required to be binded to the scope of the source Observable

https://github.com/ReactiveX/rxjs/blob/master/src/operator/do.ts