Exactly what the title says. I've played about 3 hours of this game now but still have no clue what defector missions are for. Do you get those defectors after beating their mission? Is there a perk that you get? When can you use them?

Are you asking how to access those missions or just what the missions are for? To access them (because I didn't realize this until I had like 4 of them available), you can hover over and click on liberated planets/bases/sectors/dots on the zoomed out map. There will be a little tooltip window that says something like "Territory Jazz wants to assassinate Snitch Nightly with a wrench, a silenced shotgun, and a recharging glitch trap."

I don't know if you unlock anything (other than maybe liberation points) but I'm pretty sure the defectors themselves retire immediately after you finish their missions.

Not quite sure, but it seemed to me that it unlocked more characters, and maybe some gear? Only done three so far, as that is how many stations I have, but I think I saw something about "unlocking silenced guns in shops" on one.
They seem pretty cool even if they don't give you anything, and are great ways to try out things you might not want to try out with your current character. After one of the ones I did I now know I can use a shotgun to get back onto a station after being thrown out for instance.

Also, @bbalpert: the names in this game are very awesome, and very "Friends at the Table" :D

I think they're mainly for self-contained challenges, with a slight possibility of getting the defector as a playable character later.

Pretty sure they are to encourage playstyles you wouldn't normally use while being infinitely repeatable. Just a good way to try unique tactics as much as you want, with no risk of losing a character.