Here is why you might need an agency: HARDLY ANYONE IN CHINA SPEAKS OR UNDERSTANDS ENGLISH.

Teaching English to China's students without the services of a Chinese teaching assistant is usually like pulling teeth. That is realized by the local government of Yuncheng of Shanxi and many of its educational institutions. If a prospective FT is unable to snag a post at a top-ranked Chinese university, where the average student's English level would make it easy to teach English efficiently and effectively, be he (or she) would be well-advised to work for a Chinese agent who provides a t/a. That's the best way for both an FT and his/her students.

Now, some schools will tell the Chinese teacher of English of each class to remain in class to assist during an FT's lesson. That comes in second-best to the Yuncheng way as the FT is not co-operating with only one t/a throughout each teaching week.

My posts regarding TAs aren't well-supported on this forum. That tells me that there are too many FTs in China here for sightseeing and having a good time rather than for serious teaching. China doesn't really want backpacking types, even educated ones, to teach EFL. Instead, China craves experienced teachers, but it's having great difficulty attracting them in sufficient numbers.