I am huge fan of Terra Mystica, so naturally I was drawn this iteration. So why would anyone I have this along with TM in my collection you ask, personally for me for these 2 reasons,
1. It has an interesting modular setup and
2. It plays excellent with 2P compared to its older sibling.
AoI borrows few aspects from Gaia Project and my wife prefers AoI to TM for some reason. One aspect I absolutely detest in this game is the addition of books, it honestly wasn't necessary and its a design bloat in my opinion, it adds no wow-factor to an already amazing design and its just there as an additional option for you to score some VPs.
Either ways, in the end this is still the TM game I/we know and love, although not a must have if the above mentioned reasons don't appeal to you or already own TM & Gaia Project.