KMP is an umbrella technology that lets you share common code and compile it for different platforms, depending on your needs. It can include Kotlin/Native, depending on your targets.
If you don’t need your library or application to be developed for multi-platform, you don’t need KMP.
If you need to compile only for Linux, for example, you just need Kotlin/Native. But if one day, you want to port that code to other platforms, you need to create a KMP project out of it.