why if you have your ender chest with you anyway? i mean, whats the point?

I loved the Cyclic mod (a Forge mod for 1.12.x MC), it had something similar, although not a book, but the Ender Bag. The only issue I have with this, is that you have to open the book and activate the summon/dismiss text, would be much easier to just right click the book to summon the chest (like with the older version, the Ender Chest summoner - fishing pole with carrot).

@ ReAnti: yes, there's the Shulker Box, but you have to carry it with you all the time, and when you die and don't get to it in time, or it burns in lava, you'll loose all items in it. But with this, you can store items in the summoned Ender Chest, and if you die and somehow happen to get an Ender Chest (8 Obsidian, Eye of Ender), or another summoning book (with a simple command), there you'll get all your stuff back, you won't loose anything. The shared inventory of all Ender Chests is the key, if Shulker Boxes had this feature, this mod would be kinda useless. But for now, it's usefulness is priceless.

no hate but, how is this useful?

We Have shulker boxes,
But, i know this is more survival friendly but we already have shulker boxes