Your hard drive stores all your software, data games, applications and operating system when your computer system is powered down.
When you turn your machine on the computers processors load up the operating system that you will be using ( usually windows and definitely in the case of this how to_ Since you hard drive is responsible for loading and saving your games and apps and data the faster the speed of your hard drives the better. An SSD is had drive that does not use a disk but innstead uses chips to save your game and load your game when the system is powered down.
Computer chip hard drives are much much faster than the older disk based storage systems. An SSD especially an NVMe type hard SSD drive system will immensly speed up the execution of your games like World of Warcraft.