A monarchy is generally more traditional and conservative and thus also offers a lot more stability. A monarch is able to unite the people better as he is the head of a nation, not just of a political party. Ideas and ideologies come and go in republics, monarchs are untouched by such storms. Monarchies are also much cheaper than republics and decisions can be made a lot(!) quicker instead of failing after years and years of planning and discussing - republics usually tend to be slow, with exceptions of course as this is a very broad term. Thus monarchies tend to be stronger and better defense wise. Corruption has less leverage on the monarchist state, not least because monarchs have to be moral as their job is literally for life. They are also more educated since birth basically as, for example, common people who are able to vote besides not even understanding what they are voting for.