A wheel needs a hole in the middle for an axle. A jug needs to be hollow within to hold water. A house needs empty space to accommodate people. Emptiness is necessary for something to be useful.
Usefulness of emptiness
Imagine writing on a piece of paper without empty space, or listening to music without pauses. A skilled orator knows when to pause & stop, to generate dramatic effect.
When you seek, see what is there, & understand what is not present, and you might reveal the interdependent relationships of all matters.