To be sure, all EU countries can be expected to benefit indirectly from banking union via a more stable financial system in the participating countries. But let me stress here that it would be in the interest of all if as many countries as possible decided to join.
Banks domiciled in countries that opt to join will enjoy the reputational gains from being subject to the same supervisory standards as their euro area peers, which might for instance dampen risk premiums on their debt. Obviously, this might encourage a number of Central, Eastern and Southeastern European countries which are not (yet) part of the euro area to join banking union.