The ability to leverage more native capabilities than JS can. A more generalized, less platform specific format for executable code already seems to be a goal for iOS and Android. Android of course has always used Java (well, Dalvik technically) bytecode, and Apple recently has started pushing their LLVM-based bitcode to reduce dependence on specific architectures.

If WebAssembly does a good job of exposing features needed for high-performance code, it could become feasible to move everything to WebAssembly and ditch support for native code on both platforms.