A binder transaction behaves sort of like a syscall, in the sense that a client process can immediately, synchronously transfer control to a server thread, rather than just enqueueing a message to be processed whenever the server gets around to it.
This enables Android to separate many of its components into different processes (at different privilege levels), and use binder for RPCs that are on the "critical path" for user interaction, without incurring impractical amounts of overhead or latency.
