The only reason for Russia to expand was for power.
It started when it unified into a state in the year 882 A.D. Then, when they began to have problems that other states were going to invade, it started to expand to protect the core of the country.
This early Russia was very unprotected and vulnerable. There was no mountains or oceans to protect it from invaders. The only real resource was pure manpower.
Centuries later, in the Great Duchy of Moscow, Russia greatly expanded.
Eventually, Siberia was conquered. Even though Siberia was "Unconquerable."
After the Expansion
Russia had Siberia. It was 1000 miles of inhospitable land that an entire army had to go through. Then they had to go through the Ural Mountains. So from the East, Moscow couldn't be reached.
That doesn't negate the West side though. That is still a grassland with nothing to stop them. As you can see, Russia still has problems.