Point one feels like table stakes for doing freelancing seriously. The third point is also very important. If there is a hiring freeze or other budgetary issues

Point one feels like table stakes for doing freelancing seriously. The third point is also very important. If there is a hiring freeze or other budgetary issues, they may still be able to pull contractors in through existing agreements with other firms. Plus those firms have already gone through the procurement process which covers legal agreements, etc. So instead of having to go through that full process again (which can be extremely time consuming in some organizations), it's much easier to funnel freelancers through an agency with whom they already have all the arrangements and agreements approved. From the businesses point of view, would you rather deal with a couple larger providers or manage individual contracts and relationships with potentially dozens of individuals?