What does Apple gain by killing IDFA?

Apple benefits from killing IDFA on a few fronts:

1. User privacy. This is their most public-facing benefit to killing IDFA. They use every possible opportunity in their marketing to show off just how much more privacy-focused Apple is versus their main competitor, Google. 
2. Hurting non-App Store user acquisition sources. Apple has long benefited from controlling the narrative around apps, and they want the App Store to be the main discovery tool for new apps. This gives Apple control of the narrative of the type of apps they want used on their platform. Developers will want to be featured on the App Store, and the App Store Editors tend to feature apps that promote Apple-centric technologies, like Sign In With Apple, Apple Pay, In-App Purchase, Extensions, Wdigets, Apple Watch support, and other integrations that make it hard for a user (or developer) to switch to other platforms
3. Reducing the profitability of in-app advertisements. App developers selling digital content earn money from two sources: in-app purchases, including subscriptions, from which Apple takes a sizable cut, and in-app advertising, which Apple does not make any money from. By making ads less profitable, developers shift towards in-app purchases and subscriptions, which grows Apple's services revenue. In addition, Apple markets their devices as providing a premium experience, and advertisements lend themselves to a poorer user experience.
4. Driving more revenue to Search Ads. Killing the IDFA makes non-Apple UA sources less able to track and measure installs, making it harder to measure ROAS margins and requiring companies to have lower CPI or higher LTV in order to be more certain they are making a return on their investment. By making non-Apple UA sources (*cough* Facebook) less appealing, developers may turn torwards spending their UA dollars on Apple Search Ads instead, which helps Apple's services revenue. (I believe Apple Search Ads retains the ability to do campaign-level attribution, though the targeting and creative controls are far less robust than other tools)
5. Hurting non-Apple SSO. Somewhat adjacent to killing the IDFA is the new Tracking Permissions Prompt, requiring users to say "yes" to an extremely scary prompt, in order to use functions like Single-Sign-On (Sign In with Facebook, Sign In with Google). This directs developers to prefer to use Sign In With Apple, further locking developers and users into the Apple ecosystem.

There are a few other knock-on effects here, but these are what I believe are the main core reasons behind IDFA deprecation. 

Also of note is the new App Store "Data Used to Track You" metadata display, which follows pretty much the same points as above.

https://developer.apple.com/app-store/user-privacy-and-data-use/