Where is the Combine-framework used?
What are possible use-cases? Is it worth learning it? Or is a legacy-technology?
Combine is Apple’s version of the RxSwift framework. The why use rx? page explains places where you would use RxSwift. They are the same places you would use Combine.
I would not recommend learning Combine unless you want to learn functional reactive programming. If you don’t know what functional reactive programming is, then you probably don’t need to learn it. Apple has made no major updates to Combine in the past several years. They created the Observable framework to remove SwiftUI’s Combine dependencies. They also have an Async Algorithms collection to move developers away from Combine.