According to Osho, outer and inner are not separate; what happens outside continually shapes the inside. Speech is the mind’s second wing: inner chatter rehearses outer talking. By refraining from speaking, you withdraw half the mind’s support, removing the need for rehearsal; gradually the inner noise subsides, ripening into silence, meditation, and the flowering of realization.
Keeping your mouth quiet helps your mind quiet down, because much of your thinking is just practice for talking.