Some claim these devices work great (there are a bunch of them).
Technically the 5V on the USB output should (but on sometimes isn't) filtered by the DAC and should be no problem.
It could help if a USB output (phone or tablet or laptop) isn't able to supply enough current for some reason.
Measurements indicate it usually doesn't change much if anything on the analog output when the USB port is capable of supplying enough power.
This also can be DAC dependent as not all DACs draw the same amount of current.
Short answer: Usually doesn't do anything but in some situations it might be useful.
Improve sound quality ? Nope, ensure proper operation... in some cases yes.