This is an optional device but becoming more and more popular, it fits where a hard drive goes but because there are no moving parts it’s a lot quicker so in effect it functions more like the memory in your computer, similar to the old days when you could configure some of your systems memory to work as a RAM drive.
You can install the operating system on your solid state drive and just use the hard drive for storing data. The popularity of solid state drives is probably due to them falling in price. They consume less power than a standard hard drive and some people have reported more life from their battery when swapping to a solid state drive. Definitely not an essential, but nice to have if you can afford one.solid state drive