The corp ships and stations act as corporation police force. This police force acts if a member is attacked.
So a corp needs to have corp ships / stations acting as police at key planets, because the corp members will not join forces by default.
For future reference, the corporation will have more assets like stations, facilities etc. that will consolidate the corporation power on a planet / solar system. Those assets will protect all corp members and will be protected by all corp members in return.
I hope you get these things added soon then.