PJI - Phase Jump Inhibitor What's the point?

Hi there. I've been playing the game since it came out, but found that the PJI isn't that useful or at least it doesn't seem that useful. At first I though it slowed down ships coming towards the planet, but from the text it seems like it slows ships down going from the planet with the PJI to another one. This doesn't seem that good since an enemy can just destroy them and jump to the next planet or if there is only one shrug it off.

Can anyone tell me where this might be used instead of just using the money somewhere else? And if I'm wrong, please give me suggestions!