What, in your opinion, was the need for the reunification of Ukraine with Russia in the 17th century?

After the feudal strife, the war with the Golden Horde, the territory of Ukraine came under the rule of the Commonwealth (Poland). The attitude towards Ukrainians in this country was appropriate. Discontent grew among the people and as a result it led to an uprising, which was led by Bohdan Khmelnytsky. At first, the Cossacks won, but after the defeat at the Battle of Berestets, it was decided to convene the Pereyaslav Rada and ask for help from Russia. Russia helped Ukraine by accepting the left-bank Ukraine into Russia and starting a war with Poland. Therefore, we see that Ukraine had to return to Russia in order to get rid of the power of Poland.