I would think it would be similar to any "neutral nation". In the sense that neutrality is only an illusion. A country like Switzerland was neutral because of their impossible terrain and their trained militia, they held a distinct advantage that made their conquest, 1)nearly impossible 2) requiring immense sacrifice and pain on the part of the aggressor and thus 3) not worth the effort. The simple honest answer is so they can continue to remain neutral. Gundams are super-weapons, the nuclear bombs of their day. If Orb were to not build them and remain "neutral" there would come a point where if an when one of the other two superpower's fell to the other, that they would be helpless against the winner - assuming the world wasn't destroyed by the conflict which I will address in a second. Mutually Assured Destruction, if you attack me I will retaliate to the full extent of my ability, causing you immense damage and equal pain to my own suffering. If I perish I shall mortally wound you with my last breath. Further the looming possibility total annihilation of both superpowers by each other hand's and the rest of the world along with it was a threat to everyone including Orb. It would be a threat that would be diminished by a further third party, with equivalent weapons, who could enter on either side giving a distinct advantage and tipping the balance of power. Thus Orb would act as an anchor, making sure that neither side went off the deep end. In the end it's all about self preservation.