Russia suffered numerous invasions across its western frontier throughout history. Napoleon (1812), Nazi Germany (WWII), Germany and the Austro-Hungarian Empire (WWI), England (1854), Poland (1609), and more.
Russia always lost more soldiers than the invading armies did, and the Russian battle strategy was to retreat slowly from the enemy, while destroying all crops and cities on their way so the invading armies could not use Russian resources to supply their battle. The Plains leading to any key Russian cities are so far inland, this strategy was very effective. By the time invading armies reached key cities, they were tired, cold, hungry and had no food or shelter to take over.
So, after WWII, and nearly 25 million dead soldiers, the Soviets wanted to prevent further invasions. They therefore kept territories along their western frontier they had conquered as a "buffer zone" between them and the west. That way, any invasions would be fought there, leaving Russia untouched, and allow Russia to use soldiers from those countries as fodder for the cannons, and preserve Russian lives. I hope this helped you!
