The article demonstrates how you can run your existing test suite on big endian with a few simple commands. Or you can just wait until someone actually wants to use it there, they run your program or test suite on their actual big endian machine and then you get a one-line pull request for the place you forgot to use the macro.

Adding other architectures to your build system also tends to reveal nasty bugs in general, e.g. you were unknowingly triggering UB on all architectures but on the one you commonly use it causes silent data corruption whereas one with a different memory layout results in a much more conspicuous segfault.