Darkest Dungeon – Is There a Gameplay Purpose for the Backtracking Penalty?

Both the Backtracker trait and the additional stress penalty refer to walking backwards within a hallway after having walked forward. I couldn't find an official source for this, but it's relatively easy to verify manually that walking backwards through an entire area causes more stress than exiting, reentering, and walking forwards in the opposite direction. (If you want to verify this, I recommend using throwaway characters)

While the game does not contain any explicit descriptions of afflictions per se, you'll have to use external resources for that, there is a limited set of things that an affliction can do.

A: Cause additional stress, in response to:

1. Start of the turn to everybody else.
2. When another hero is hurt by an enemy to that hero.
3. When they are hurt by an enemy to everybody else.
4. When another hero attacks an enemy to that hero. only skills that deal damage are affected

B: Deal damage to friendly targets, at the start of their turn (does not consume their turn).

1. Deal percentage damage to self. ~10%, round up
2. Deal percentage damage to another character.
3. Deal weapon damage, plus bleed/blight if applicable, to another character.

C: Act on their own

1. Pass their turn.
2. Move forward one. Does not consume their turn.
3. Move backward one. Does not consume their turn.
4. Use a random ability. If no ability is available, will pass instead.

D: Refuse help Help counts as:

1. Heals or buffs cast in combat. Targeted abilities will allow you to cast another ability. AoE abilities will waste their use on the afflicted hero, but still get cast. If a targeted spell fails in this way, the hero becomes unable to be targeted this round.
2. Camping skills used outside of combat on the hero. The camping skill is wasted, and the points are lost.
3. Eating food or using consumables. This does not disable you from trying again, so it's only a cosmetic effect (as you can simply try to feed, bandage, antivenom, etc. them a second, third, etc. time.
4. Switching places with the hero. Characters can't stop another character from moving past them, so if your hero that wants to move past has 2 movement, you can move two steps instead. Also, this does not stop heroes that use skills to move to switch positions, such as the graverobber's Lunge attack.

E: Reduce statistics

Generally, each affliction will affect statistics in various ways. Most will for example lower resistances by 25%, Health by 10%. When your character is afflicted, you can view their status screen to check what stat changes have been applied.

Some afflictions may also increase statistics (most notably the Flagellants' Rapturous state.

Not every affliction can have all the effects. In general, the name of the affliction is a big clue as to which effects are enabled for that affliction. For example, a paranoid induvidual is likely to refuse help, and act on their own, but won't hurt themselves or others.

Most affliction effects seem to have about a 25% chance of occurring whenever they can occur. To catalogue the exact effects, you'll have to look into the game files.

One event per hero

A hero can only react to something once, even if their affliction has two separate possiblities in response to an event. It is however possible for two different afflicted heroes to react to an event. For example, if a masochistic character is crit by an enemy, they may comment how much they liked it (upsetting everyone else), and then the fearful hero could complain about how hopeless the situation is, causing a triple load of stress on the two remaining heroes.