Peace surely is a good reason, yes. But there are other reasons too.

Justice is happiness according to virtue.

Many of our most serious conflicts are conflicts within ourselves. Those who suppose their judgements are always consistent are unreflective or dogmatic.

Now the good of political life is a great political good. It is not a secular good specified by a comprehensive doctrine like those of Kant or Mill. You could characterize this political good as the good of free and equal citizens recognizing the duty of civility to one another: the duty to give citizens public reasons for one's political actions.

Public reason arguments can be good or bad just like other arguments.

An injustice is tolerable only when it is necessary to avoid an even greater injustice.

A scheme is unjust when the higher expectations, one or more of them, are excessive. If these expectations were decreased, the situation of the less favored would be improved.