<p>I have come to see an Installer class item in Visual studio. Why they have maintain an seperate item for Installer. Do they create any custom installers ?</p>

The Installer class can be used to configure items such as performance counters and message queues as part of the installation of your code.  They can be included with any assembly and the most basic way to install components related to an assembly is to use InstallUtil yourassembly.dll which would contain your code and custom installers.  It is good practice to provide an uninstall option for anything added in this manner.
See here fore more information
http://msdn.microsoft.com/en-us/library/z3xc61bs(VS.80).aspx