It's just one of the things that are known to be able to help people who stutter. That's why it's so often recommended, and that's why people study to become speech language therapists. - My case isn't paradigmatic of stuttering, so I can't really tell you much of use in that regard. But I was in speech therapy at an early point - probably too early - and then again at a later point - at which point I had sort of already arrived at the beneficial conclusions I could about my stutter, so I kept that part of it up (it's moreso something about mindset, and not so much about specific techniques, althought I did work on those for a bit also). Again, that's just me, and we can't use that for anything general. That kind of abstraction needs more data and such. And that's what speech therapy is based on.