I think the request to interview the whistleblower comes from the desire to see how much involvement Mr Schiff's office had in identifying and encouraging a person willing to initiate an official inquiry based upon things they heard from a friend who heard from a friend, for the continuing purpose of invalidating an election.
All they needed was someone willing to say they heard something through the grapevine and then they could manipulate public opinion through a willing press and a large percentage of the population willing to believe gossip as proof. Public opinion is not enough to secure a victory in the impeachment process but it will be invaluable in the next election. That is the point. The question is will it succeed?
Edited to add: I see the Washington Post has gotten ahead of this question by
publishing a whistleblower timeline detailing his/her thought processes, motivations, conversations with friends and steps taken to complete the process of blowing the whistle. It occurs to me that if the whistleblower has provided interviews to the press, he/she should also be available for questioning under oath.
Unfortunately, the story I linked is behind a paywall and I could only read another sources highlights.