Let's be honest, we are all tired of the many sources for information for raids and encounters outside of the game, and most of all, currently there's not a single objective and accessible way to see what debuffs/buffs were on the boss at the moment you reach a new mechanic or the type of attack you missed by chance to see what it was. Have to either stop paying attention at the environment to read the description, record the pull or take various screenshots to read the boss's buff/debuffs after the wipe it's bad and strains the time everyone want to spend progging.
Then there came the idea that it would be really useful to have an Encounter Codex, which for every new mechanic the player actually sees (or most likely dies to), the debuffs that went off or the type of attacks would be logged with their description in a neat window that can be easily accessed and inspected in-game. And the best of all, after that very long time you spend away from the fight, and you just want to remember certain things without having to rely on extensive video guides for a single micro information, its there, catalogued, in-game.
There could be more features, but to not break the feeling of progression and puzzle solving, I believe certain types of information should only appear after you've seen the mechanic (mostly buffs and debuffs), and newer content would have a period until SE would add more descriptive informations (more spoilery) to avoid breaking the sense of progression and blind discovery on the first couple of weeks and during world prog when the content is fresh.
EDIT: not to be confused with a Death Recap (another QoL that the game also desperately needs), which is different since it only shows the name of the attack the player received and how much damage it made. That does not provide information on what the debuff actually does.