Why should you pay attention to the physical states of reactants and products when writing elquilibrium constant expressions?

Answer is: because pure liquids (shown in chemical reactions by appending (l) to the chemical formula) and solids (shown in chemical equations by appending (s) to the chemical formula) not go in to he equilibrium constant expression, only gas state (shown in chemical reactions by appending (g) to the chemical formula) reactants and products go in to he equilibrium constant expression.
For example, equilibrium constant expression Kp for reaction:
A(s) + 2B(s) ⇄ 4C(g) + D(g).
will be: Kp = [C]⁴·[D].
But for reaction A(g) + 2B(g) ⇄ 4C(g) + D(g), will be:
Kp = [C]⁴·[D] / [A]·[B]².