What would PCIe SSD this big be used for and who would actually buy one?

So I don't get the use of it? Where would it be used to make the purchase worth the price?

You are having a mindset of home use, where you are the only user on a system. When a device's resources is split amongst multiple users, resource needs scale accordingly. 2 users using a 100Mhz CPU effectively only get 50Mhz each. 2 users using 10MB/s of a storage space speed effectively only get 5MB/s if they both use it simultaneously. If you want to support more users, you need faster stuff.

Let's use Facebook as an example. Think of the sheer number of transactions that a site like Facebook does a day - how many clients are simultaneously posting and/or reading at any given moment. It's likely in the billions.

Any major site that is used by millions or even billions of users is going to need all the power it can get.

I understand it's not designed for home use, but that is a huge amount of money to spend on a server SSD which have been ruing fine with standard hard drives until SSD's came along.

"It's ruing fine, so why improve?" - Currently, private businesses that aren't monopolies need to constantly grow otherwise stockholders become unhappy and then your company stagnates and gets overtaken by other companies.

So nothing is ever "ruing fine," it's a matter of "how much power can we get for the amount of money we currently have" or "how many users are we adding per interval X and what do we have to spend to keep up with that growth curve?"

SSDs provide so much more IOPS and speed over hard drives that of course they will be used to enable applications to run faster and support more users.

$15K for 3TB of PCI-e attached SSD (which is VERY fast and VERY large for SSD) is really not terribly unreasonable. Maybe a little pricey, this is MLC NAND and not SLC though.

A spiing mechanical hard drive is 75-100 IOPS according to Wikipedia. So this is at least 5000x as fast. But it's not 5000x as expensive.