1. You want to run POSIX-compatible software in a context that existing kernels won't support. 2. You want to understand how POSIX works. 3. You want to wri

1. You want to run POSIX-compatible software in a context that existing kernels won't support. 2. You want to understand how POSIX works. 3. You want to write a kernel but not design it, and POSIX offers plenty of references. 4. You want to optimize something about POSIX implementations and needs a bottom-up approach. There are plenty of valid reasons.