I provide an outsourced sales & marketing function for IT and technical companies.
Without coming across as an advert the real benefit is for those companies who could not justify a full time salesforce, but when they need one, they need a good one. Just like hiring a lawyer.
One of my clients is a software house and a big project will usually keep them fully occupied for up to 18 months. So if they employed me full time, when I brought in a deal, they'd have to sack me! And firms like that are not usually the best at cold calling new business and conducting effective sales campaigns.
Where it works less well from my viewpoint (but well for the client) is when they are a bit commercially naive to start with. I can go along and introduce some effective procedures and get the ball rolling. Then after the first few sales the company realises that a) it now knows how to do it, b) it really ought to be a prime function of the business, and c) it's a cheap solution when you're not selling much but it gets quite pricey when you're successful.
In other words - as a client there is very little risk and an awful lot of benefit in giving it a go - as long as you have an escape clause.