Placement versus employment.
A staffing agency is in the placement business. It finds candidates, presents them and typically bills against a markup or a fee, and its involvement thins out once the person is in the seat.
An employer of record is in the employment business. Connext signs the contract, runs payroll and benefits, holds compliance in the local jurisdiction and stays responsible for the person for as long as they work for you. The comparison turns on what happens after week one.
A staffing agency knows how to find people, and if you want them on your own payroll that is exactly the service you need. Choose Connext when you would rather not carry the employment, the HR and the compliance yourself.
Follow an engagement from the day you sign it to the day you judge it. At each milestone the two models do something different and at the last one, the BPO wins.

Figures as published on the source comparison page.