Since there is the option for the first two it just felt weird not to be able to do it with the other two ^^
Could be interesting for people who want to finish a boss without getting hit for example. They probably don't want to replay the whole game to get there again (sure, not saving after the boss would work too, but would still be nice to be easily accessable in some way too)
I totally understand. We are potentially discussing a bossrun mode where you could select battles later. For now we added replayability if players needed to "practice". Storywise I don't want every battle to be replayable because if enemy disappears or similar in the next scene.