
The reason for both of them is simple. How much work do you have for them? Do you really have to build and design a website that's going to take a few days to do (depending of it's complexity) or you need them for the next year with big workload in order the finish the desired issues or to solve the problem?
How much money you need to give them also counts. Some on the freelancers on the freelancer platform are cheap or very cheap while a full time employee is more expensive than them.