There are of course regional differences due to different laws and customs, even between different states within the US. But in general a temp agency gives the employees a long term job with them but then hire you out for short term jobs to other companies. This is how a temp agency can hook you up with a long term job. It is also quite common for companies to hire some of the temporary workers at the end of their contract. So it can be seen as a sort of a trial period where the company expect most not to be needed afterwards.

The list of jobs you provided is not the typical temp jobs. The demands for these jobs are pretty consistant so it makes a lot more sense to hire someone full time rather then the higher costs of temp workers. Where you do see a lot of temp workers are in phone sales, customer support, modeling business, conferance staff, etc. where the number of people needed can vary extremely based on various events. Where you do see temporary workers in other jobs it is often to replace a worker on temporary leave or in some cases to handle the increased demand around hollidays.