On the app, you get a shard for every deck you scan. After 18 decks are scanned you get a key. Do we know what the keys do yet?

If you hit the info button is gives you some very vague info on them

Info button only available on the website, not the app (at least the Android version I use).

Here's what it says:

Quote

Collect your own fragments of the mysterious Æmber that runs throughout the Crucible. To earn this in-app currency, master the world of KeyForge by registering your decks, elevating your Archon's power at official tournaments, and more! Æmbershards can be redeemed for prizes or even earn you access to special events!

You’ll be able to exchange them for FFG flavored pencil toppers.

You will get a special ffg christmas card.