The example a few lines down that had to be changed shows exactly: 
    nreplace ~str:"bar foo aaa bar" ~sub:"aa" ~by:"foo"

If you're asking for some specific scenario where someone would depend on the order of replacements, I can't come up with an example off the top of my head, but I'm certain that somewhere there's people for whom this is an important difference, and changing this function will introduce a probably hard-to-debug error in their code.