If, you water your plant with nutes in it. The plant will uptake the nutes (mostly at night) by the roots, it does this while the rest of the plant is resting. The plant is setting itself up for all the work it has to do tomorrow. The nutes will travel throughout the plant and give it the nutes it needs where it needs.
The nutes do not take care of the roots. The nutes work their wonders down here too. The roots are there for stabilization of the plant and nute and moisture uptake.
No matter what anyone says, a plant cannot tell the difference in the nutes they uptake. N is nitrogen, whether it comes from piss (it will burn your plant) or from Peter Pan's nutes mix. They don't care. You, however, may notice a difference in taste.