A more thorough explanation:

The main-purpose rule is a doctrine in contract law that states that if a promise to guarantee another's debt is made primarily for the promisor's own benefit, then the statute of frauds does not apply and the promise does not have to be in writing. This is also known as the main-purpose doctrine or leading-object rule.

For example, if a business owner promises to pay a supplier's debt to a bank, but the main purpose of the promise is to ensure that the supplier continues to provide goods to the business, then the promise does not have to be in writing.

Another example is if a parent promises to pay for their child's college education, but the main purpose of the promise is to ensure that the child is able to get a good job and support the parent in their old age, then the promise does not have to be in writing.

These examples illustrate how the main-purpose rule applies when the primary purpose of a promise is for the promisor's own benefit, rather than the benefit of the person whose debt is being guaranteed. In these cases, the promise does not have to be in writing and is still enforceable in court.