Here's why it's important:

* Orderly arrangement: Atoms are not randomly scattered but have a specific, predictable positioning. This creates a long-range order within the solid.

* Geometric pattern: This arrangement is not just random order, but follows a specific geometric pattern that repeats throughout the crystal.

* Three-dimensional: The pattern extends in all three dimensions, not just a single plane.

So, while an orderly geometric arrangement of atoms is a key characteristic of a crystalline solid, it's not the only one. The repeating, three-dimensional nature is also crucial.