For money, of course. Donetsk region is full of good quality coal and benefits coming from controlling the Black Sea are definitely very tempting.

> Why would Putin want to annex a country full of people who hate him?

Please change your optics. People never matter in Russia/Soviet Union. Even, if there will be 70-80% of people who are dreadful opponents to "new goverment" then what they would do? Take it to the street? Strike? Please remind yourself how it looked in the Kazakhstan in early January 2022 (you will get shot, just because you left your home to buy some food) and in Belarus 2021 (mass protests, thousands of people being thrown in jail for no reason, very often beaten, many found dead there). Coup d'état? Poor joke, especially if we look at the latest events in Africa (for example Mali), where armed Russia's Wagner forces helped to take over the whole country.

Edit: As a person who lives in Central Europe, your question made me sad, because it means western mass-media is crap these days - they cannot explain a very silly thing why this ongoing conflict for 8+ years takes place.