If one thread is using relative paths, and another is doing a chdir-based traversal (as using the nftw function, for instance), that first thread's accesses are messed up.

This is why POSIX now has various -at functions; the provide stable relative access.

The former allows you to design a coherent system. a lot of design questions which are annoying (“how do I access config data consistently, etc) become very clear.

It also makes C more productive. If global vars and static locals are unbanned, features like closures become less important.