One of the questions is why hire a freelancer to do your web design?  I think the simplest answer is flexibility and cost efficiency.

As a freelancer I have a great amount of flexibility in what type of services I can provide.

You maybe a large company with an IT department but just need an extra person for a few months to work on an Intranet site for maintenance work orders.

You may be a web design company owner that needs a developer to assist with a technical aspect of project for one of your customers that is not in your knowledgebase.

Developing a relationship with a freelancer expands the knowledge base of your company but avoids the cost associated with an traditional employee. Need a programmer for a project that lasts 2 weeks but you don't need them till the next programming project comes through the door. A freelancer can be the answer. Keeping an employee on staff for an occasional need is not cost effective but using a freelancer is. You pay a freelancer when you need them but don't pay them when there is no need.

A freelancer gives your company the flexibility to work on projects that may exceed your current knowledgebase in a cost effective manner.