What are the main reasons for the Russian-Turkish wars in the second half of the 18th century?

The first and most basic reason is the strengthening of Russia’s positions within the Black Sea. Russia needed access to the Black Sea, because:
– this would allow to strengthen the southern Russian borders in order to stop the constant attacks of the Tatars from the south;
– across the Black Sea it would be possible to pave the way to all kinds of European markets, which would significantly contribute to the development of the Russian economy;
– in this way it was possible to extend their influence to the Balkan Peninsula.