Draz is right; let's flesh it out a bit.
One answer is: Value-added features. The open-source product does X, but the product from a vendor does X + Y. This is a tempting business model for vendors because you can build feature Y once, then sell it over and over with relatively little additional cost per sale.
The other answer is: Trained talent, continuously on tap.
If you're a company and you use an open-source product internally, there will be times when you want the advice of an expert. Something has broken and you need help finding the problem. You want to improve performance and you want advice on how. New features have come out in the open-source product and you need someone to explain what they are and how they might benefit your company.
What do you do? You can look for free advice online in forums or IRC. But the best advisors don't work for free very often. And those who do work for free won't prioritize your request for help over all the others that flood these kinds of forums. Your company may have tens of thousands of dollars riding on this. You would happily spend many extra dollars to get a definitive answer now. You need a well-connected matchmaker to help you connect your big check with the appropriate talent.
You could look for a well-regarded consultant. But identifying the good ones and testing them takes a lot of preparation. Even if your address book has been primed with the contact info of the best consultants, they have backlogs. Just getting them on the phone may be difficult. Worse, what if trouble strikes and you call the consultants and find that they're all booked up, or retired, or they've switched to consulting on iOS development and abandoned your platform?
You could find or train a brilliant expert in your open-source tech and then hire them full time to sit around waiting for the need to act. But that's awfully inefficient. And then the expert gets bored playing Tetris all day and gets hired away or goes off to found a startup, and you're back on square one.
Enter the support firm. They'll sell you "support contracts" (pay advisors/troubleshooters by the hour, with a minimum number of hours per month) or "professional services" (pay the developers by the day, week, or project for larger-scale projects than are given to "support". Either way, you get folks who meet a minimum standard set by the support firm, and who often have the ability to escalate questions directly to the experts who built the tool in the first place.