As we return to "browsers with an editorial opinion" market differentiation is based on that opinion. Apple has been pushing privacy really strongly to move people to the Apple ecosystem. As the "browser that won't let you be tracked or compromise your privacy" they have an opportunity to further "choke" the data/advertising streams of competitors.

It is the advantage of having a weapon that you can use that damages your competitor but not you. Chrome started out that way[1] as a way to attack Microsoft (and to some extent Firefox)

When you see things like Tim Cook saying he will "end" Facebook, you might reasonably ask, "Wait how could he possibly do that? He doesn't control the web." And you would be right, with the exception of understanding that Facebook makes its money on data extraction[2], and a lot of Facebook users are also Apple device users. So on the Apple device Apple can build a browser that lets their users access Facebook while taking away as much data as possible from Facebook. This damages Facebook revenue, forces them to invest in counter measures, and doesn't change Apple's revenue model hardly at all.

As a result, from a business perspective, I think Apple could justify investment here in a browser that puts forward a point of view that makes it harder for Google and Facebook to make money.

[1] FWIW I worked at Google when Chrome was launched and understand that there was the 'open standards' narrative but there was also a pretty obvious 'hurts Microsoft/Apple' narrative. So the technical reasons to invest aside, my opinion is that the real reason any large enterprise invests in anything is because they have a reason to believe it will make their business stronger, more difficult to attack, more profitable, or with luck all three.

[2] I think the term 'surveillance capital' a bit too inflammatory.