Consider the differences between the bilateral contract and the unilateral contract. In the bilateral contract, we have the offeror willing to exchange a promise for another promise ("I promise to pay you $1500.00, if you promise to paint my house on or before Saturday."). In the unilateral contract, the offeror is insisting on performance for a promise ("If you complete painting my house before Saturday, I will pay you $1500.00).
Why does the law make such a distinction and why would a party to a contract choose one type over another?