I would have loved to see x32 take off, but alas it did not.

CONFIG_COMPAT provides the same pointer memory savings, but not the register, instruction, and syscall benefits of x32 or x64 allowing unmodified 32bit userspace environment to run on a 64bit kernel. A good compromize for legacy programs and pointer-heavy applications that can survive with 4GB virutal address space.

CONFIG_COMPAT is also useful on many embedded ARM systems that happen to have 64bit cores, but small amounts of RAM, the userspace memory savings are usually worth it.