Why do you need to join a union? .... I'd say: represent yourself.
Roman, I admire your confidence (which also comes out in your reference to seniority - clearly you feel you would need no such protection). This suggests that you have not yet met the worst that the aviation industry might throw at you. The main reason for unions is that individuals can be played one against the other for the purposes of reducing the T&Cs of everybody.
One of the few jobs in which we repeatedly see strongly non-union types become union joining and tolerating types is aviation. We may be naturally right of centre in our political disposition but there is nothing quite like being systematically ripped off by a malevolent employer to encourage a pragmatic embracing of an effective union. Union = joining together (for mutual benefit and protection).
If it is anciliarly benefits you want at the moment, it may be IPA for you. If it is protection & anciliary benefits it may be BALPA.