If your chosen trading method is a Personal Service Company (PSC) you may, depending upon the position you undertake, require a contract of employment in addition to a contract for service with your end user client.
If your PSC employs other individuals as your business expands, or you are a CIS contractor, then contracts of employments will be required for your employees.
If you work through an umbrella company, you will need a contract of employment. This will include key issues regarding your rights for holiday pay, sick pay and expenses etc.
Advice is recommended, and Freelance World provide a contract review service to enable you to make an informed choice when agreeing terms and conditions for both contract for service and contracts of employment. Employment law is an extremely detailed area and expert legal advice is recommended.