One theory is that it helps stifle subvocalization. When you’re silently speaking to yourself, the brain actually sends signals to the tongue, lips and vocal chords, triggering very slight movements, almost undetectable. This is called subvocalization. It could be that immobilizing the tongue and the lips by clamping the tip of the tongue between the lips sends feedback to the brain that it needs to shut up so you can concentrate on what you’re doing.
It’s interesting that meditators are told to keep the tip of the tongue pressed up against the roof of the mouth, behind the front teeth, and to keep the lips closed. I think this is for the same reason.