It depends on the nature of your contract when you hire the photographer.
Some photographers used to charge a low fee for shooting the event and make their money selling prints and digital rights to the images. This is common particularly in events where there are a lot of people who might like images. This used to be very common in weddings.
Other photographers charge a much higher fee and basically work as private contractors so the resulting images are owned by the person who hired them. This is more common in commercial work, and the contract I used most often for advertising and editorial work as well as corporate clients and events.
Between the two extremes is all the minutia of reproduction rights management. Are the images licensed to you for non commercial use only? Are there limitations on where and how they can be displayed? Are you allowed to edit and make derivatives?
Always discuss this with the photographer before the event while you're negotiating the price. Odds are they will have several options and pricing schemes available.
I've had problems several times where a client did not read the contract I sent over and as a result thought they had purchased rights to the images that would have cost them thousands of dollars more to actually acquire. (For example, a restaurant that had hired me to do some promotional work tried to sell the images as stock photography after they went out of business.) I eventually had a short summary section added at the top of my contracts that enumerate what they are required to do, what they are paying for, and what physical material or digital rights they are purchasing.
 As prints have become less common and digital presentation has become more common this has dwindled considerably. Though as a result the base cost of hiring most experienced wedding photographers has increased considerably.