What is the significance of using 'if and only if' in Philosophy instead of simply 'if'?

Often in Philosophy (and Maths) we may come across the phrase 'if and only if', regularly abbreviated as 'iff' and denoted by an arrow pointing in both directions. <-> Logicians refer to this as a bi-conditional and it actually holds a great deal more meaning than it may appear. The statement p iff q, means that both p and q follow from one another. Not only does p entail q, but q entails p.
For example, the statement 'I will take an umbrella if it rains' (p --> q) entails that, if it rains (p), then I will take an umbrella (q). However, the statement, I will take an umbrella if and only if it rains (p <-> q) entails not only the above but also that if I take an umbrella, it must be raining.