There are two sides to the answer. Russia largely wanted to protect its naval base in Crimea and its access to the black sea. Crimea has wanted to be independent from Ukraine ever since the Soviet Union fell. It actually used to be semi-independent from 1992 to 1995 until Ukraine overthrew its government.

These factors affected each other. Russia knew that the population would be accepting of Russian rule so taking Crimea would be relatively easy and would protect its naval base from the new western-backed government in Kiev. I honestly don't know whether Crimeans would have preferred independence or joining Russia. But given what we know now about how the Donbas republics fared, joining the Russian federation was the correct move regardless of what people might have wanted.