It will begin collecting this data unless you opt-out. It hopes that such information will lead to greater insights about their user base.
This, in turn, will help it sell better targeted ads, something its business model is heavily dependent on.
While the company currently collects data on what users tweet and who they follow and engage with, the information is limited to Twitter's own ecosystem. Monitoring a user's app downloads will give Twitter a wider view and more competitive information.
It will also help Twitter figure out what it should be doing with its own app, or if it should be building its own products.
Imagine if this existed years ago. Twitter might have seen Snapchat exploding in popularity. It could have tried to buy Snapchat. Or it could have built a Snapchat killer.
While Twitter will be able to know which apps you have installed on your phone, they won't have any visibility into what you're actually posting on those apps, and can't gather data on anything inside them.