I don’t even understand why Putin needs to do it
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.
Putin invaded multiple countries when Obama was president, and Obama did nothing, actually less than nothing as Obama and Clinton as secretary of State "reset" relations with Putin.
Putin does this because he needs to scare his population into obedience. If the Russians do not heed to Putin's power, his government will fall. There is a possibility (although small) that Putin's government will not surivie due to the unpopularity of their response to the coronavirus.
Russia does not have free and fair elections, but Putin still directs Russians to vote every so often. There have been some
local elections in which the opposition party has made gains.
The point is that Putin wants to scare his opposition from putting forward effective candidates, and preventing an effective from campaigning