The Soviet annexation of Moldova (Bessarabia and Bukhovina as it was known in 1940) was largely to do with both increasing Soviet influence over Romania, and gaining a foothold onto the Danube River, as its mouth is near the region (this is also tied into the first point). Moldova, which is arguably populated by ethnic Romanians (sorry to any Moldovan nationalists) served this through what has been coined the "Piedmont Principle," which meant that the Soviet Union intentionally acquired ethnic minority groups on its borders that had strong majorities on the other side, and used them as a means to foment dissent in the non-Soviet territories (Piedmont played a major role in the Italian Unification). And as the Danube was the centre of control for access to the Balkans from the rest of Europe, and easily gave access to Central Europe, it was imperative for the Soviets to try and gain control of it.