Simply put, Siberia had furs. Much Russian expansion in Siberia involved establishing relations with local tribes in which they were obligated to provide a tribute of furs to the Tsar.

Russia thought of many of the Slavic states as "theirs". They believed they were some mother country to them and must protect and have them. Kinda weird, but makes sense as they are all in the same area.

So, Russia obtained Siberia, it was only until the age of oil where they realized how important it was. During Stalin's reign he would send "bad" people to go work in the mines there. He did this by having a secret police that made sure people didn't insult the government.