they are an agency and a fantastic example of why the company should have it's own staff
Unfortunately this is also a way to absorb most/all of an airline's operating profits.
I recall the withdrawl of Alaska Airlines direct handling at Tucson, Arizona airport, cutting back to a handling agent (quite unusual in the US). They had just one daily flight in and out at midday, on a 30-minute turnaround. For this the station had 14 full-time Alaska Airlines staff, and they didn't do any work for anyone else. There was also a dedicated check-in desk, office space and ramp equipment. It must have absorbed so much of the revenue.
Agree you do need to retain one of your own management to oversee things carefully.