Employment contracts have a number of benefits to you as an employee. For instance, your employer will be restricted in the ways he or she can terminate you. So, there is no way an employer can simply terminate you on a whim or due to a clash of personalities. If that happens, then your contract may protect you and be used to fine or otherwise punish your employer.
Employees and employers are both bound by contracts in that they require you to act fairly and in good faith. If an employer is not being fair or if an employee is not treating an employer with respect, then the contract may be able to be used in court to show that one side was not acting in an unacceptable manner. For instance, if your contract requires you to come to work on time daily, but you never show up until halfway through your shift, then you would be acting unfairly. Likewise, if your job description is IT services but you're being asked to perform duties related to driving or teaching, you would be able to point out that you're being unfairly asked to extend your duties.
Contracts can restrict your right to work after you leave a company, but only within certain realms. For instance, if you work at an IT company, you may not be able to work for a certain amount of time with another IT company to protect sensitive information. Once that time passes, there is no reason that you shouldn't be able to go back to work at a similar job.
Contracts often require that you give notice before you quit your job. That means that you should always plan accordingly and make sure your employer has plenty of time to replace or fill your position.
Source: Cim Search, "Pros and Cons of Employment Contracts" Dec. 18, 2014