The Asus ZenFone 3 zoom comes with a microSD card slot which supports cards up to 2TB according to the official specs on asus.com . This feature might not be that necessary for the 128GB, but it's certainly become more useful if you plan to purchase the models with either 32GB or 64GB built-in storage. Obviously a photography-oriented phone like the ZenFone 3 Zoom must have a this feature because it allows users to capture and store a large amount of photos and videos. This becomes even more important if you are among those who love shooting 4K videos or intend to shoot in RAW file format. Shooting just a few minutes of videos can really start feeling up the storage pretty fast. When you have a microSD card slot, you can even carry two or more cards and swap them in case the card is out of space.
This is useful for people on vacation or a long trip who intend to shoot a lot of videos or for professionals shooting lots of photos outdoors and rely on high storage to store the digital multimedia files on. Transferring media to an external device when you are outdoors is really an annoying task and although there are some external storage device solutions out there, what's better than just carrying a few microSD cards and swap them when needed. Micro SD cards are also quite cheap now and you can buy a 64GB microSD card for as little as $20.
You probably want to know if there are any 2TB microSD cards, and from what I've read, those cards don't exists just yet. I've searched many online eCommerce retailers like Amazon and newegg and found none. The only solution is to buy a portable storage solution and transfer the data from the card to the external storage and delete the data from the card and reuse it. The ZenFone 3 Zoom is definitely future proof as far as its micro SD slot capacity support goes, but I think by the time a 2TB is out, you'll probably have a newer device.