You can make costumes and use them in your current project. You cannot then save them, start up a different project, and use them again.
Something you'd want, if, for example, you were to use that same NPC ship in the follow up mission. You wouldn't want to have to recreate the uniforms and the notable NPCs like the captain.
I thought "importable projects" was supposed to cover this (mostly), but that's not on the roadmap as far as I can tell...
Although, if you're creating a large project, you're going to want/need backups - and I don't see any easy way to do that without saving it locally. e.g. accidentally delete a map, and there is no way to recover it etc.
For that reason, I do think the developers should seriously consider how to allow a player to save content to their PC, and from that potentially allow them a local 'archive' of editor content? (the savefile should contain the editor settings to create the object, not the object itself - thereby making it useless to hack).
I do like the list of upcoming features though - lots of fun to be had there!