To comply with section 24(2) of the DSA. You need to publish that information about your number of EU users.
Currently there's no strict rules on how you calculate or even necessarily the information you publish. Other than saying based on your calculation your number of users is below 45 million (in the EU). In the future there will be delegated articles on how the calculation must be done.
Obviously the regulation was made in comitee with team of lawyers but I expect the justification for "Why should a corporation have to disclose MAU?" is something like:
* The majority of businesses will with minimal work be able to tell the difference between having under say 4 million EU usere and having 45 million eureopean users so they'll know if they're in a range where they should be concerned about the regulations.
* If they're getting to a size where they have millions of users (in europe alone) they should have the resources to put in place logging/policy to figure out if they're approaching the 45 million EU users with more confidence.
* By making them publish a statement saying they've done what they believe is due dilligence on making sure the rules don't apply then enforcement becomes much easier. Auditing only needs to be done randomly on a handful of sites that declare they're just under the limit and on those who look like they are taking the piss saying they're well below the limit or seem to be growing but keep saying they're just under the limit declaration after declaration.
So these numbers are shown in Apple's case because it's in a companies best interest to show they're acting in good faith in working out these numbers (Especially since in Apple's case they're trying to differentiate the services so that they can try to reserve the right to not apply the DSA to some of those services in a possible future). Stating a number (or that you're well below the user limit) implies you've done a real calculation (i.e. I only get 44 million site visits from anywhere in the world every 6 months so I can't have 45 million EU users) and in the case of an audit where it's not the case shows you were acting maliciously/with gross negligence/with a calculation tha needs to be changed, making any enforcement much more justifiable.