The most common reason is to do optimizations such as replacing strlen("hello") with 5 or open-coding strlen (or, more commonly, memcpy or memcmp). If you're linking with a non-conformant strlen (or memcpy or whatever) the usual thing that happens is that you get standards-compliant behavior when the compiler optimizes away the call, but you get the non-conformant behavior you presumably wanted when the compiler compiles a call to your non-conformant function.

But the orthodox answer to such questions is that demons fly out of your nose.