XAMPP is a server distribution. It include Apache, MySQL/MariaDB, PHP, Etc. Essencially XAMPP bundles third party software into one setup. XAMPP offers a control panel that allow the control of those applications.
If you want to host locally a wordpress blog, test scripts or perform any webserver task than you should install it.
XAMPP Denial Of Service is just a program that connects to the internet to retrieve a list of public XAMPP servers and performs a Denial Of Service on those servers. This tool doesn't require user interaction to perform the Denial Of Service, just the execution of the file.