name of a plant, the roots are used for various purposes
芨 is the name of a specific plant whose roots are used for various practical purposes. Its meaning is tied to that particular plant rather than a general concept.
芨 combines the grass radical 艹 with the phonetic component 及, suggesting a plant whose name sounds like 及. The exact historical development is uncertain.
The grass radical 艹 sits above 及, like a plant reaching up. Picture a useful plant whose roots are pulled up and used.
For キュウ, imagine a plant whose roots are used to make a strong glue: glue -> キュウ, and the plant's roots provide the sticky substance.