Are the roots of biennial plants used by humans after the fruits are harvested?

No, they are not used. We harvest roots in the first year of plant development. For example: onions, carrots, beets, parsley, radishes. A root crop is a kind of “pantry” in which the plant accumulates nutrients for the winter. The next year, in spring, the plant releases a flower arrow, pollinates and fruits (seeds) are tied on it. But in the second year, the root crop becomes unsuitable for human consumption.