Here's why:

* Molecular formula: Tells you the types and quantities of atoms in a molecule. For example, C6H12O6 is the molecular formula for glucose.

* Structural formula: Shows the arrangement of atoms within a molecule, including the bonds between them. This provides much more information about the molecule's shape, properties, and reactivity.

Example:

The molecular formula C4H10 could represent two different molecules: butane and isobutane.

* Butane has a straight chain structure.

* Isobutane has a branched structure.

Their different structures lead to different physical and chemical properties, even though they have the same molecular formula.

In summary: A structural formula provides much more information about a molecule than its molecular formula alone.