A flat tax bills everyone at the same rate. Concessions can be made for people below a certain income level.A Flat tax simplifies the tax code overall making it less of an ordeal to pay.
A flat tax means that everyone gets to keep more of their income, and this includes businesses. More income means more spending, which means a stronger economy. A flat tax will help curb tax avoidance. If we have a tax rate like this companies in America will be encouraged to stay and companies abroad will be attracted to the lower rates.
Lower taxes encourage Domestic and foreign businesses. It's good for the economy.