Kotlin Native has nothing in common with SubstrateVM (Graal is a runtime, Substrate is its part which allows AOT builds). These tools cover different use cases. K-N is used to create a multi-platform or stand-alone native applications, Substrate is used to pre-compile JVM applications for faster startup.
The actual question is why do you need one of those tools. If you believe that you will get faster performance (not startup time), then both will be slower, than JVM. Maybe you will be surprised, but JVM performance nowadays is better than most native implementations and Graal has some new optimizations (some of them work only on EE though).