Control Internet Usage
One of the most important reasons why proxies are widely used in corporate networks is that they provide control over what employees are accessing and using on the Internet. For example, many companies ban certain websites so that they are not accessed by employees during work hours. Also, they may ban adult websites, popular social networks or time wasting websites.
Forcing employees to access the Internet only through your company's proxy also means that you have detailed logs of all the websites & content they accessed, when, for how long, etc. Detailed usage reports are available about each employee and the company can easily identify misbehaving users.
Bandwidth Savings & Improved Speed
Another reason why companies use proxy servers is that it helps them save precious bandwidth. Proxy servers can compress traffic, cache files and web pages from the Internet and even strip ads from websites before they reach your computer. This allows companies to save bandwidth, especially when they have hundreds or thousands of employees, accessing mostly the same popular websites (e.g. CNN news, New York Times, etc).
When a web page is accessed, a proxy server can store it and, when the next person requests it, it first checks if the page has changed. If it hasn't changed, it forwards the local copy without re-downloading the whole page. This both saves bandwidth for the company and makes the loading process faster for the next person that requests the same resource.
Proxy Servers Can Improve Security
Proxy servers may also have a role in improving security, especially when used in business networks. They can be set to block access to malicious websites that distribute malware and they can also provide encryption services so that your data is not easily sniffed by third-parties that want to get their hands on it.