a) you may pray/venerate/communicate with your ancestors
b) ideas are often jumbled and need to be refined. positive thoughts, negative thoughts can flash across the mind. coralling your ideas into a cohesive theme helps strengthen and reinforce the ideas you wish to keep. the breath controls the thoughts, but until that process is perfected; choosing words is taking definitive action about those thoughts.
c) "prayers", "words" are a more concrete phase in that creation process; the "words" represent a clear formulation/recipe for the ideas. "words" are really the first action taken towards realizing ideas. the next step is to take physical action around those words. "words" and thus "prayers" are more potent than ideas.
even if you realize that you are praying to yourself, or that which is in yourself; the "words" are empowering those thoughts.
additionally, if you are communicating with "spirits", those are recognized as external, but on another plane. so the outwards gesture is recognized as stronger than thoughts alone.