What are the main advantages of a multinational state?

The advantages are:

Attractiveness for tourists. Traveling, for example, in the republics of Siberia or the North Caucasus is more interesting than in any mono-national country where almost everything is the same everywhere (examples are Poland, Portugal, Japan, Albania).
Different cultures complement each other, it just so happened historically that some peoples tend to herd livestock better (Tuvans), while others are better engaged in intellectual and entrepreneurial activities (Armenians and Jews).