What is the doctrine of consideration in contract Law, and what was the impact of the case of Williams v Roffey Bros

Doctrine of consideration is based upon the idea of ‘reciprocity’. A promise in contract law should not be enforceable unless the promisee has given something in exchange for the promise (i.e. Remuneration, counter-promise etc)e.g. If I promise to transfer you £100, leaving aside equitable rules of contractual estoppel, this will not be enforceable unless you give me something in return for this promise.
To satisfy the doctrine of consideration, something of recognised legal benefit must flow from the promisee. Certain things do not constitute good consideration. For example, the orthodox position was that performance of a pre-existing contractual obligation is not valid consideration. see case Stilck v Myrick Ship captain hired sailors for Charter Party, but when journey became trecherous sailors wished to abondon ship. Captain promised them more money if continued. When completed journey Captain refused to pay the addditional wage. Contract was not however enforceable. Agreement to perform the sailors pre-existing contractual obligation was not good consideration for the promise to pay more money.
However, this orthodox position was altered in the seminal House of Lords case of Williams v Roffey Bros: Similar Fact pattern:A carpenter was contracted by the defendants to complete a building contract but underwent financial difficulties and so requested an additional payment.The defendants, anxious to avoid the time penalty clause of the contract, agreed to pay the carpenter an additional sum. On reliance of this promise the carpenter finished a number more of the houses, but defendants failed to provide the additional sum promised.Judgment:This time the promise for additional payment was enforceable.Ratio of the case is that a promise to perform a pre-existing contractual obligation can be enforceable if the promisor can be said to have received a 'factual benefit'.On the facts this was the timely completion of the building project and the avoidance of the time penalty clause for the contractor. Case marked a shift from focus on legal to factual benefit.