Convert last remaining streaming hold outs - Apple has high quality audio as standard, vs Spotify who still haven't implemented it even as a premium feature. The other high quality audio streaming platforms are probably too far away from the classical demographic (though, maybe Qobuz could) so it's a market where they can - potentially - own the market.
If you can get those folk to see the value of Apple Music, and they have an Apple TV (I bet there's a strong crossover here) and get them onto Apple One family plan, then you get kids/other family members using Apple Music.
So taking classical makes a lot of sense.
But yes, there's potentially also a physical side to it. I'm assuming that if Apple starts acquiring classical labels they will pretty much let them continue to operate as a record label - but with the luxury of perhaps not having to worry quite so much about top line economics.
Not a classical label, but another "premium" label I'm friendly with did a limited edition boxset of a particular artist and sold 1000 copies at about $400 - $500 a shot in under 12 hours. So doing premium releases is big money - but also very costly to manufacture, and when people are paying that sort of money they expect a REALLY great unboxing experience. Which, I guess, kind of aligns with what Apple does elsewhere.
How does that square with Apple wanting to put the music from this label in an app? Upsells to collectible media?