Suppose you hire an interpreter directly. If that interpreter you have scheduled arrives late or doesn’t show up, then you’ll be in a bind. If you hire an agency to send an interpreter, one will show up. If the one assigned to the assignment becomes ill the morning of or his/her car breaks down (or anything like that) the agency will send a replacement. Agencies have many interpreters working for them, allowing for them to find a replacement at a moment’s notice.