These aren't wild guesses, but I also don't have inside information.

1. Browsing history. We know that Verizon is tracking it for their gain: https://www.wired.com/story/verizon-user-privacy-settings/. It seems reasonable that T-Mobile and others don't want that door to close on them.

2. Video streaming management. Carriers typically restrict video streaming on some/all of their plans to certain resolutions. For example, I think most American carriers limit video streaming to around 480/720p at 1.5Mbps or less unless you have bought a premium plan. VPNs often get around this and I know that my carrier can't detect Netflix access through iCloud Private Relay. Right now, iCloud Private Relay doesn't proxy app traffic, but it could in the future.

3. It looks like mobile carriers are looking to get into "edge cloud" stuff. Verizon has been pushing this and they recently emphasized this in their 5G Ultra presentation. If traffic is going through iCloud Private Relay, buying expensive "edge cloud" services from Verizon is a waste of money since the traffic would be leaving the network to go through Private Relay.

3a. Netflix ships "Open Connect Appliances" that ISPs can hook into their network to serve Netflix content. If your traffic is going through a proxy, you start accessing the content on a server farther away. This mostly doesn't apply given that Private Relay only does Safari traffic, but one could see Private Relay expanding to apps in the future.

4. I think there is a certain knowledge of what is using data that can be helpful to carriers. For example, I worked for a university and they wanted to set different QoS for things like peer-to-peer file sharing vs. web browsing. The university didn't want to punish P2P tech or anything like that. They just wanted to make sure that P2P usage didn't overwhelm other users and uses of the network. Likewise, it could help the university spot patterns like viruses/bots that might be using a lot of network traffic.

4a. I think this can also play into how companies position their offerings. For example, T-Mobile has introduced features like "Music Freedom" and "Binge On" that allowed unlimited audio streaming and video streaming before unlimited plans were a thing. They surely did analysis of network usage of those features before introducing them. You can look at how much video streaming users are doing and then model how much data would be used if you limited it to 480p (including accounting for an uptick in usage due to it being unlimited). However, if you don't know how data is being used, you lose the ability to spot patterns that might be opportunities.

4b. It makes sense to want to offer different QoS for different services. If someone is using FaceTime, you want that to be a good experience. You don't want to prioritize a speed test over someone's FaceTime call. You don't want to prioritize downloading from YouTube over a FaceTime call. That YouTube video can be buffered and if you know that you've transferred 15 megabits worth of 1.5Mbps video, you kinda know that the user doesn't need the next 1.5 megabits of video for 10 seconds.

4c. I know that a lot of people want their connection to be an unbiased dumb-pipe, but I think that people only want that because they tend to see crappy stuff from companies looking for money. Seeing it from a university that only wanted to give people the best possible network experience feels a bit different. QoS can be a positive thing and a dumb-pipe isn't always great.

I'm a bit surprised that T-Mobile would go this route at this time. iCloud Private Relay doesn't proxy app traffic at this time and I haven't seen that they have a similar browsing-history program like Verizon's. Still, there are reasons to want to be able to understand your traffic both for business reasons and for a better customer experience. Again, I'm surprised because it seems like the reasons today are slimmer. I think the Netflix OCA use case is a good one since it reduces network usage in a way that simply helps the parties involved, but wouldn't really be possible if the traffic first went via another external server.

I'd emphasize that nothing here is to say that T-Mobile is doing the right thing. It's just to bring up areas where a company might want to know more about its network access patterns. Some of that can be used for good like the Netflix OCA system or giving higher QoS guarantees to FaceTime. Some of it can be used for bad like knowing using browsing history for advertising.