In terms of what new information of substance can the whistle-blower bring to the fore that is not already confirmed or refuted by more closely engaged sources, there is no purpose and nothing for the whistle-blower to offer. Since the allegations have been corroborated, and more, it seems, the purpose of the whistle-blower is more key for the defense of Trump - as a distraction from the facts and evidence in the case.
Consider, that when the report was initially made, the defense you'd hear from the GOP and conservatives was that this person had nothing of value to offer, as their perspective was not "first hand." Now, the counter-argument was that the report and examination of the claims led to more direct corroboration, so bringing the issues to light was important, but the whistle-blower, himself/herself and what they had to say (already in the report), was less important than offering a map of where to look and who to talk to.
So, the same people, who previously, before any witnesses were heard, were discounting the value of anything this person had to say, now think it is very important to talk to them, even though they don't have anything more to offer than what was in the report, and certainly less than other sources who have spoken to the same issues.
But as a distraction, the defense is to hope to throw ancillary issues that have no probative value into the mix, and make a muddled mess of things.
Supposedly, it is important to find out if this person had a partisan agenda, if they hated Trump, if they ever lied about anything in their lives, if they had ever "leaked" information.
None of that matters.
If I hate a person, and see them shoot someone dead in the street, and I report it, maybe if my claim is the only piece of evidence, it might be looked at with skepticism. If my reporting it leads to the police talking to other witnesses who saw the events, pulling camera footage that objectively documents the events..... how does my own feelings about the person impact the veracity of the claims and facts? It does not.
This person followed the documented procedure for raising an issue, by the book. To eliminate bias and false claims, the process has a review by an independent, objective party - the Inspector General. If the Inspector General finds that the claims are "credible and urgent," THEN it is supposed to be forwarded. The IG did find this, so, again, whatever their motivations, whatever their goals, an independent party without the same agenda, bias and feelings assessed the claims to be credible.
Whether the person ever "leaked" information is irrelevant, because this was not a leak - the proper procedure was followed by the whistle-blower. Whether the person ever lied about anything is irrelevant, as these claims have been independently vetted and corroborated.
So, if you are searching for the truth about what happened, and are seeking useful information, the whistle-blower has nothing of value to offer to the process, at this point. Their valuable input started and ended with the filing of the report, and with the contents of the report.
If you want to create an imaginary conspiracy of some sort as a lifeline to those desperate to deny the facts in evidence, then the whistle-blower has value as an imaginary boogey-man.