The idea is this: in the current system of welfare benefits, they are phased out with income. At those income levels, this phaseout acts as an implicitly high

The idea is this: in the current system of welfare benefits, they are phased out with income. At those income levels, this phaseout acts as an implicitly high marginal income tax, which creates a disincentive to work. If you give everyone basic income with no phaseouts, you accomplish the same goal of welfare transfers without those ill disincentive effects.