It's because they need to start collecting more first-party data from users who land on their site. This is a result of Apple (and others in the future) blocking third-party cookie tracking.
They are doing this SOLELY because of the need for audience creation, marketing attribution, and ad revenue.
They sure do; however, digital media and social in particular, absolutely rely on significant investment in their audiences, attribution, etc in order to drive more revenue and thus higher CPMs. More traditional media (such as OOH, Print, etc) all rely on very high-level metrics such as daily traffic volumes and lack of direct impact evidence in their attribution of value.
This is why Facebook is SO very against what Apple is doing with iOS14+, particularly with cross-device and cross-app tracking opt-in, because they know it will decimate their ability to do what they do today.
Bingo. They need this for user-level measurement and targeting. Wouldn't be surprised if this also supports part of their audience extension work with twitter audience platform as well.
They are doing this SOLELY because of the need for audience creation, marketing attribution, and ad revenue.