Since There are no standard rules for application(legacy applications such as EXE's) installation, uninstalling and upgrading, each application has its own unique rules. Hence to provide a standard way in which a uniformity can be maintained throughout the installation,uninstallation and upgrading microsoft introduce windows installer technology .
For proper management of shared components.
To maintain system stability which may get break during installation, un-installation and upgrading of application.
To reduce high maintenance cost by saved time and effort through automated installs.