This is because bandwidth costs are high if you want to lease a line into your own premises, to handle the load from the Web. These solutions provide good value as well as control - if you like your own space and your own customised server. Data centres and bandwidth sellers often incorporate these into the package. Standard features you should see in a data centre include fire and security features as well as diesel-driven backup generators. Depending on the vendor, the prices for such services may vary.
Web traffic is often hard to predict, and it is safer to err on the side of caution.
For example, if there is too little bandwidth to your Web server, you may end up with frustrated customers. Reliability is important, of course. You do not want an operator that is known for lag time and does not respond promptly to your requests. Prices vary among bandwidth providers. Here's a list of websites where you can check on the details.