If you own Sony's PlayStation 4 console in any of its forms, then you're going to want a PlayStation Plus membership for a few reasons, not the least of which is the free games Sony gives you every month just for being a member. The other major reason is multiplayer gaming, as you'll need PS Plus if you want to play with your friends and others online.
Being a PS Plus member also gets you some additional discounts on games and other content that you can buy on the PlayStation store. Sony is always having sales on games and other things in the store, but if you're a PS Plus member you're getting even steeper discounts than non-members and if you buy games often you won't want to miss out on some of the better deals you'll get.
As a monthly price PS Plus is $10, but if you buy in bulk you get a discount. Normally a 12-month membership is $60 but because it's Prime Day the membership has been knocked down to $40, which saves you $20 off the original price and gives you even more of a savings on the month to month cost as that breaks down to just $3.33 a month.