iOS let's you modally present any view controller, but if you want the presented view controller to not cover the whole screen or modify anything about its presentation or transition you have to use the Custom View Controller Presentation API's.
This can be cumbersome, specially if you do it multiple times in your app. Presentr simplifies all of this. You just have to configure Presentr depending on how you want you view controller to be presented, and the framework handles everything for you.