Why did Russia need access to the Baltic Sea?

Because in this way it would be able to increase its sphere of influence, as well as establish trade relations in the north of Europe. The first attempts to break through to the Baltic Sea were made by Tsar Ivan the Terrible. True, his war ended in failure, since at the same time an internal conflict was taking place in the state. Further, the Russian state was on the brink of disaster, and therefore, only in the period of Peter 1, such a goal became achievable.