Inline interfaces - are exist only at compile time. All inline interface functions are inline.
Inline interfaces are just guaranty existence of some functions set that will be dissolved by compiler.
As example, where they can be used.
I have some data class (packet) that format recognized by some network transmitter.
Other classes, if we want to sent them via transmitter, have to implement some interface, and pass to some converter to packet format function.
Interface here just a contract.
For better performance could be great completely replace call to interface function with there body, just like inline functions do.