A contract is what lets you sue someone over a private transaction. That's what it does, that's all it does. If for whatever reason you're not willing to bring a contract dispute to court, then your contract doesn't do anything and you wasted your time writing it. Contract = right to sue for breach of contract.
In order to sue someone, you need to be able to describe what damages have been done to you. The goal of a lawsuit is for the responsible party to 'make you whole,' i.e. pay you back an amount equal to the damages done to you.
In a contract dispute, the 'damages' of breaking the contract is equal to the 'consideration' of fulfilling the contract. In other words, the promised consideration is the actual thing that you can sue over.
If there is no consideration, then there are no potential damages, and there is no potential lawsuit. And since the only point of a contract is to enable a lawsuit, a contract that doesn't do that isn't a contract.