Fa may introduce the answer or consequence of a condition. It does not merely join unrelated acts; it marks the consequence as dependent on fulfilment of the condition.
«إن دخلت الدار فأنت طالق»
If you enter the house, you are divorced.
Condition: entering the house.
Consequence: the legal effect stated in the historical divorce example.
Role of fa: linking the consequence to fulfilment of the condition.
It is not enough to spot fa by sight. One must identify the conditional relationship it expresses in the actual wording.