The current conversation about a wealth tax was jumpstarted by Thomas Piketty, who argues that the natural tendency of a capitalist economy is to make the rich

The current conversation about a wealth tax was jumpstarted by Thomas Piketty, who argues that the natural tendency of a capitalist economy is to make the rich get richer and the poor to get poorer (relative to each other - in absolute terms everybody should be getting richer). Basically the point of the wealth tax is to provide a counterweight to that tendency.