To diminish inequality
The concern is not in inequality but rather the increase in income inequality since the 1970s. The reason for the rise is due to the change in information and communication technology which has favored the productivity of high-skill workers. The current minimum wage is lower than the minimum wage of the 1970s due to inflation. Household incomes have become more unequal because of changes in family patterns, especially the rise of single-parent families and the rise of high earners marrying each other. Lastly, globalization has played a role but a minor one with job loss to low-wage countries.
While higher taxes on the rich can be used to diminish inequality, the share of federal taxes paid by those with high incomes is fairly high by the standard of recent decades. An alternative method of reducing inequality would be to increase government spending or actions that benefit those with low incomes. This could be accomplished by expanding direct payments to the working poor, such as the earned income credit and/or increase the minimum wage. Another way would be to increase government spending on infrastructure, which would create higher paying jobs.