Apple was one of a number of parties that fought the repeal of net neutrality protections, originally instituted in 2015 during the Obama administration. The company has a lot of pragmatic reasons for taking a pro-neutrality stance.
Things were made (mostly) clear in an August letter to the U.S. Federal Communications Commission. "An open internet ensures that hundreds of millions of consumers get the experience they want, over the broadband connections they choose, to use the devices they love, which have become an integral part of their lives," Apple wrote.
"What consumers do with those tools is up to them — not Apple, and not broadband providers," it added.
Assuming it's not reversed by the courts or political maneuvers, the FCC's repeal of neutrality could conceivably let internet service providers block, throttle, or prioritize traffic as they see fit. On a basic level, Apple needs to ensure quality of service on its devices — people dealing with slow internet access may be less likely to want the latest iPhone, download apps, sign up for Apple Music, or rent a movie from iTunes.
Apple has also had to deal with services being blocked in the past. AT&T, for example, once prevented people from using FaceTime over cellular unless they had a Mobile Share data plan, presumably because it was worried customers with grandfathered unlimited data would bombard its network.
Behind the scenes, Apple is probably worried about rivals signing deals with ISPs, or those ISPs simply favoring their own services. If Comcast decided to prioritize its own TV streaming traffic for example, that could play havoc with material sold on iTunes and Apple's upcoming original video programming.
Lastly, Apple may also want flexible bandwidth for future technology. In its August letter it complained that killing net neutrality could "create artificial barriers to entry for new online services, making it harder for tomorrow's innovations to attract investment and succeed."
The company is rumored to be developing an augmented reality headset, which will likely be highly dependent on high-speed 4G/5G cellular for functions such as Siri and navigation. Apple is also working on a self-driving car platform, which will probably be sold to third parties — if it hooks into Apple's cloud services in any way, that could generate terabytes of data per car, per day. Apple might have to spend millions on prioritization deals or risk an unusable product.