An Agency is run by a pimp who, by taking a cut of an Escort's fees, agreed or not, is controlling prostitutes for gain, which is illegal. Neither the escorts or punters are breaking the law unless the escort was underage or being coerced. Note that many agencies set up legal arrangement to seemingly get around this. But there are examples where these have not work in court.
A pimp can run an Agency from virtually anywhere. All they need is a computer and a mobile. They don't need expensive offices or indeed any business premises if the the escorts have to supply the premises. If the Agency does provide premises, then the pimp is also running a brothel, also illegal for them. Agencies offer a punter no more protection from a bad punt than any other form of punting. Just because they call themselves an Agency and have glossy pictures on their site doesn't mean the escorts featured exist or offer a good service.
The only way of finding this out is by asking punters you find credible. Agency reviews can't just be blindly believed as the pimp could have posted the reviews themselves. UKP is the only reliable place to check out reviews. Don't place your trust in an Agency just because their site looks professional. All that shows is that the pimp, or someone they know, can put a nice site online. Links to pages below provide specifics.
The benefit? They might have a number of escorts on offer, so within one phone call a punt is sorted. Together this can make booking easy. For some punters they can build up some trust with the pimp. Lastly, if things go wrong, the agency may provide a refund, or an alternate escort. Overview condensed from here.
Firstly, there are mainstream in-call agencies. These are agencies with primarily non-exclusive genuine escorts offering both incalls and outcalls. The page on this wiki has 95 agencies of this type.
