Are there any benefits of using subcollections in firestore?

Question

I have a subcollection for each doc in the users collection of my app. This subcollection stores docs that are related to the user, however they could just as well be saved

Answer 1

The only potential technical advantage to sub-collections that I can think of relates to document size -- it allows you to omit a reference to the parent element. (i.e. for each related rootDocument -> subCollection, there is a single pointer from the root collection document to the subcollection).

Subcollection

/messages
|
 --> /messages/tags

When tags are a sub-collection then they don't need to save the messageId because you access the tags through message document:

collection("messages").document(messageId).collection("tags")

Root collection

/messages
/tags

With a root tags collection each tag would need to store messageId. This basically flips the direction of the pointers.

collection("tags").whereEqualTo("messageId", messageId)

Summary

Sub-collection: pointers go from one -> many

Root collection: pointers go from many -> one