They are mercenaries recruited from the criminal class. For example, if a Russian soldier is let go for criminal offences such as stealing/assault/murder etc., there is another employment opportunity within Wagner. The leaders, while elite and highly skilled, are also criminals.
Having a strong, brutal mercenary army like this would be a useful tactic for Putin, particularly in the first few months of conflict. But as a dog can unexpectedly become vicious and turn on you, this type of soldier would need to see a tangible return for their effort to avoid turning on Russia. Unfortunately, the criminal mercenaries have had enough and are marching towards Moscow.