Uninstallers, sometimes referred also as Installation managers, are tools that give more flexibility over software installations on your PC.
Basic operating system's uninstall capabilities are rather limited -- in Windows, most programs leave traces of themselves behind even after a "complete" uninstallation. These traces include empty directories, abandoned registry entries, various settings files and temporary files all over the hard drive. In most cases, Installation manager / Uninstaller monitors your PC all the time, notifying the changes each software makes to your operating system and HDD. Once you initiate an uninstallation process, Unistaller "remembers" what traces that particular program has made to your computer and can remove all those traces in one go, keeping your system less prone to hiccups and, very often, running much smoother as the all excess baggage is kept out of the system, even after installing and uninstalling dozens of software items.