The short answer is "Because it is both cost effective and result effective.” These are the rationales:

• Specialized: The IPs bring in specialized skills and knowledge that is highly targeted for a particular situation that the client is tackling. Such skills and knowledge are either expensive to maintain or simply non-existent within the client organization
• Transparent fee structure: Independent Professionals (IPs) are typically engaged on a project basis. This is particular well suited for clients who have a clearly defined budget and task.
• Flexible: IPs work on flexible schedule according to the client's pace and the project progress
• Short term: They are easier to hire and fire - and are not a long term commitment

To put it simply, a cost effective way to add specialized resource when you need it and where you need it. They complete one off or small, regular tasks that do not require a full time employee. The other major reason companies like freelancers is that they save money. If a company employs you they have to pay sick pay, holiday pay, redundancy pay and employer's national insurance. But if they use you as a freelancer they don't have to pay any of this.