It seems that it is a way of claiming the CS as part of your sphere of influence, without actually donating them credits. At least, I don't see the point of using it if I have donated credits, as they will then be my Friend or Ally, which makes the point quite obvious.
However, if by pledging to protect, it makes a possible enemy wary of influencing that CS themselves, then it is worth it. The AI often do this as a way of protecting CSs they might later wish to associate with, especially, I have noticed, Greece and Siam.