They can probably understand what we go through on a daily basis much better. They have direct similar experiences themselves.
Does this make them better?
You could have an SLP who comprehends totally what you’re going through, but could be totally incompetent in doing therapy.
An SLP who stammers, also needs to ensure she/he doesn’t get ‘too involved’, in the story of the person they are helping.
This might not be so much of a problem for an SLP who doesn’t stammer, at least in the context of stuttering.