A contracting party at common law has a general right to assign its rights without any consent or approval from the other party (unless by its very nature the right is personal). An assignment clause may be included in an agreement to exclude or limit this common law right. In order for the assignment of rights by one party to not be exercised unilaterally without the knowledge of the other party, it is common for contracts to include a provision that a party can only assign its rights under the contract with the consent of the other party.
After assignment, the assignee is entitled to the benefit of the contract and to bring proceedings (either alone or by joining the assignor) against the other contracting party to enforce its rights. The assignee does not become a party to the contract with the promisor. As the burden or obligations of the contract cannot be assigned, the assignor remains liable post assignment to perform any part of the contract that has not yet been performed.