After world war ii in 1945, what would explain why soviet leaders wanted a buffer of satellite countries between the soviet union and germany?

Russia had no secure border. Following the devastation faced by Soviet Union in WWII, they wanted to push the border away from Kiev, from Moscow & Minsk 
The Yalta Agreement between FDR, Churchill & Stalin had laid out the spheres of influence that each of the Allied countries would have, and the satellite countries all fell under the Soviet sphere.