He doesn't need to do it, but the fact is that he can do it with impunity. Putin is untouchable in Russia, and so is able to crush all opposition. And as recent history has shown, other countries will do nothing to stop him. China is an ally, the US under Trump has turned its back on the world, the EU is scared.

Every time he does something like this, and the only effect is a bit of grumbling in the international community, it encourages him to do the same again in the future. And each time he does so, it strengthens his strong-man image. We see the same behaviour with other powerful countries, most notably China's approach towards Hong Kong.