Meaning
Unilateral commitments to provide a benefit lack the necessary exchange of value to be traditionally enforceable as contracts. A gratuitous promise is a pledge to give a gift or perform a service without receiving anything in return. Because the law requires a trade of value to form a binding contract, these promises are usually revocable at the will of the speaker.
Enforceability Gap
Law generally treats a gift as something that can be withdrawn until it is actually delivered. A gratuitous promise creates no legal obligation on the part of the speaker because the recipient has given nothing in return.
Reliance Cost
Problems arise when recipients change position or spend resources based on the expectation of the gift. While a gratuitous promise is usually not a contract, some jurisdictions use the concept of estoppel to prevent a promisor from backing out if it would cause extreme unfairness. This requires proof of the specific harm suffered by the person who relied on the word of another.
The record of that reliance is often harder to prove than the promise itself.
Speakers Right
Speakers can usually change their minds at any time before the benefit is handed over.