From my understanding, it encourages reuse of phrases. You don't want to create two same phrases, one with brackets and one without, as it is a waste of phrase and increases difficulty for translators (just think of the amount of new phrases XenForo would have because of the bracket / plus thing)why ?
what is the benefit of this ?
It is much easier to change a "phrase" than to mess around in templates....
Definition:why does the text "(You must log in or sign up to post here.)" and "(You must log in or sign up to reply here.)" need brackets after all ?