It’s a JSON based mail protocol rich of new features and more efficiant than IMAP.
Maybe some of the feature which require a Mailspring account today can be solved using JMAP?
It might also attract new users because there are still very few mail clients supporting JMAP.
Would be fantastic for this to get added. Doing any mass migration/pruning/reorganising of emails with imap is a buggy mess with all providers, due to imap.