Historically Apple buys enabling technologies and IP and not marketing/market share/customers (headphone biz), which is what they would be getting here.
I doubt this is for the music service, which is rumored to be having challenges - both internally and with customer acquisition.
Beats has a popular cross platform music player app. iTunes is under immense pressure from apps like Spotify who are far more youthful, fun and social. So having a decent app for Android/Windows would be useful and Apple definitely doesn't have any experience in house.
Also just saw this comment about the app, "ATT put it on my phone without warning me or asking me if I wanted it." Existing carrier licenses to preinstall iTunes on Android could be useful.
This is a interesting point, but such contracts don't always freely transfer to an acquirer in an M&A situation. Depends on how they are written ("change of control" etc).