You would do this if you authored a publication with a small, independent publisher whom you want to protect from bankruptcy.
The decision of the Federal Court of Justice (BGH) that publishers have to return the share of VG Wort royalties they received between 2012 and 2015 to their authors has cost publishers 20--200 percent of a yearly proceed (according to a science publishing association).
Most publishers can handle this, but some small publishers seem to struggle, and these are often houses that publish financially risky work beyond the mainstream, i.e. they may be run by patrons of the arts rather than business people. Some authors have a close and mutual relationship with those publishers and would relinquish what's now legally their share of royalties in favor of keeping their publishers in business.
See, for example, this excerpt from an open letter by a dozen authors who published with Verbrecher Verlag, Maroverlag and other independent houses:
Nun wollen wir von dem für unsere Verlage schmerzhaften bis ruinösen
Ausgang [des BGH-Urteils] auch nicht profitieren. Wir haben gewiss nichts zu
verschenken, aber wir wollen uns auch nicht gegeneinander ausspielen
lassen. Es sind die unabhängigen Verlage, die unsere eigensinnigen
Bücher in einem nicht einfachen Umfeld durchgesetzt haben.
Wir werden deshalb das Formular der VG Wort „Verzicht auf
Rückabwicklung zugunsten von Verlagen“ vor dem 28.2.2017
unterschreiben und bitten Euch, das ebenfalls zu erwägen.
We don't want to profit from the Court judgement, which is painful or
even ruinous for our publishers. We certainly have nothing to donate, but we don't want to
be played off against each other. It's the independent publishers that
have made our idiosyncratic books possible in a difficult environment.
Therefore we will sign the [pertinent VG Wort form] and thereby
relinquish our royalties, and we also ask you to consider this option
While this letter is about the redistribution of returns between 2012 and 2016, it illustrates the reasoning behind the decision of some (few) authors to share their VG Wort proceeds with the publisher also in the future.
The initiative "fair book market" lists further considerations for and against sharing. Those in favor are either based on notions of fairness, or on the strategical calculation that disgruntled publishers might leave the VG Wort in the long run to form a new entity in which author's interests are represented less well.
In sum, this is unlikely to be relevant for most authors of scientific literature, but it might be a consideration for authors of literary or philosophical works at the fringes, who want to support their indie label.
Strictly based on your assumption: there is no (strong) point in sharing your returns.