because it simple...

what is user agent,http user agent data

Main content page - what is user agent.Like the user-agent string for Internet Explorer, this user-agent header provides details about your system. Alternate tokens can appear in the user-agent string for a number of reasons; usually, they identify optional features installed on your system.
This topic describes the user-agent string, which identifies your browser and provides certain system details to servers hosting the Web sites you visit. The topic also shows how to view your user-agent string, summarizes tokens used by recent versions of Windows Internet Explorer, and documents registry keys that affect the user-agent string.
Today, nearly everyone uses a web browser as their user agent. Sometimes it's necessary for a web site to understand how it is being viewed, so most user agents identify themselves by sending a User Agent String to the web site.
In short, it cannot really be trusted. 'HTTP_USER_AGENT' Contents of the User-Agent: header from the current request, if there is one. This is a string denoting the user agent being which is accessing the page.
It is a request of sorts: the web browser is requesting access to the website. Part of this request, then, is that the web browser be identified as the user-agent. This identification of the user agent contains specific information, including the application name and version, the host operating system and language.