Sometimes, especially if she has previously worked in an expatriate household, your new maid will want to do things in the manner she is used too. While this is acceptable in some instances it could be quite awkward in others. Agencies - the advantages of using an agency to find domestic staff is that they deal with all the formalities. They ensure that the Workers Compensation insurance is taken out and so on. If a particular person is unacceptable, then one can contact the agency, which will tactfully organize a replacement.
Personal confrontation is avoided and there is no hostility or loss of face on either side.
This is very important if you're going to live in Thailand for a while.