For example, a Self Doctor is effectively an Activated Bulletproof player. Many Self roles aren't particularly useful; a Self Cop probably doesn't care much about the reminders of their own alignment (although they would be able to learn if a Tailor or Framer had targeted them, so the role is not completely a waste).
The most commonly seen Self roles are the Self Watcher (who becomes aware if someone has targeted them at night), and the Self Deflector (who can protect other players from night actions via causing the night actions to hit the Self Deflector instead, and thus plays out somewhat similarly to Bodyguard).
Because a Self role typically has all its targets locked in, some moderators cause the role to be used automatically (making it Compulsive or passive depending on your point of view). This typically doesn't make a huge difference, but can help avoid issues when a player forgets to send in an action.
Use and Power
Self roles are normally much weaker than the original role, because the targeting choice is a) highly restrictive, and b) often the worst possible option (because most roles either hurt their targets or gain information about them, both things that you don't really want to do to yourself). Even when the modifier doesn't outright ruin a role, it often causes it to play very differently. As such, Self roles are mostly only used to add a bit of variety to a Theme game, adding extra (but minor) possibilities to a role that would otherwise be expressed in a simpler way.
Note that a Self Watcher becomes much more powerful if Bulletproof (because it's then informed of people who attempt to kill it). This is still, however, less powerful than a regular Bulletproof Watcher would be (unsurprisingly so; a Bulletproof Town Watcher is outright broken).