To host your own web activities in house is often an expensive overhead. You would need skilled IT personnel as well as the required hardware and other technology on your premises. Hosting companies provide all of these services and resources as part of their offering, along with other useful services such as firewalls> for generally less than it would cost to manage all of this in house.
With companies continually looking at ways to reduce their overheads without compromising on service, the use of a hosting company can provide a very viable option.