Earth’s military, especially the officers, must be trained in the art of bluster. Even UNIT isn’t immune. If invading aliens could be defeated merely by talking at them, the Doctor thinks, earth would never need his help. They’d be more than able to cope.
Sadly, the universe doesn’t work that way. Despite all the hot air the military and politicians are producing, indignantly asserting their sovereignty over their planet, words can’t sway Cybermen, or Daleks, or even Sycorax. It’s a good thing the people of earth have a little outside help in such situations. Someone needs to keep them alive.