What do Valve have to do with that fight between Apple and Epic?

Apple are aiming to demonstrate "the total size of the market for Epic's available digital distribution channels," and argues that data for Steam - as a digital distribution channel for games like Fortnite (but not Fortnite) - is essential in that quest.

Request 2, Apple argues, "is very narrow." Specifically, it states that Apple want Valve to provide "(a) total yearly sales of apps and in-app products; (b) annual advertising revenues from Steam; (c) annual sales of external products attributable to Steam; (d) annual revenues from Steam; and (e) annual earnings (whether gross or net) from Steam.".

Apple apparently initially wanted information on "all 30,000+ games on Steam over ten years," but reduced that to "436 games over six years," but Valve argue that this merely "makes an impossible task slightly less impossible.".

Valve does not disclose its sales and revenue information and projections, and Valve derives a significant value and edge from the confidentiality of such information, including by keeping it out of the hands of companies like Epic who also sell PC games."