What exactly are the Comet Shards for?

[PalmTree]

So far, I've seen that you can use them to get Shard Dust and a few districts, as well as the heirloom trade, but to they have any other purpose?

[Skinny Pete]

There's a shard store in the Nomad Wagon (lower left) where you can buy trinkets.

Absolutely buy the Broken Key, and if you aren't playing Abomination, start, because this trinket turns him from a mediocre character to a great stunner (who also deals damage if you pair the key with for instance Osmond Chains).