Akka Streams: No-arg GraphDSL.create() vs GraphDSL.create(sink) - scala

Question

The doc has the following example (only what's relevant to my question is shown):
val resultSink = Sink.head[Int]
val g = RunnableGraph.fromGraph(GraphDSL.create(resultSink) { implicit b => sink =>
import GraphDSL.Implicits._
// importing the partial graph will return its shape (inlets & outlets)
val pm3 = b.add(pickMaxOfThree)
Source.single(1) ~> pm3.in(0)
Source.single(2) ~> pm3.in(1)
Source.single(3) ~> pm3.in(2)
pm3.out ~> sink.in
ClosedShape
})
I was curious about why the sink has to be passed in as a parameter to GraphDSL.create so I modified the example slightly
val resultSink = Sink.head[Int]
val g = RunnableGraph.fromGraph(GraphDSL.create() { implicit b =>
import GraphDSL.Implicits._
// importing the partial graph will return its shape (inlets & outlets)
val pm3 = b.add(pickMaxOfThree)
val s = b.add(resultSink).in
Source.single(1) ~> pm3.in(0)
Source.single(2) ~> pm3.in(1)
Source.single(3) ~> pm3.in(2)
pm3.out ~> s
ClosedShape
})
However, this changes the return type of g.run() from Future[Int] to akka.NotUsed. Why?

Answer 1

I think I found the answer myself. Acc. to the doc:
using builder.add(...), an operation that will make a copy of the
blueprint that is passed to it and return the inlets and outlets of
the resulting copy so that they can be wired up. Another alternative
is to pass existing graphs—of any shape—into the factory method that
produces a new graph. The difference between these approaches is that
importing using builder.add(...) ignores the materialized value of the
imported graph while importing via the factory method allows its
inclusion
g.run returns the materialized value of the graph, thus the change in return type.