While I never played WoW, I have heard that many addons used a private chat as a means of inter-... whatever communication. If there is a way, people will eventually find and use it.
The "fight" between Blizzard's boss design and addon creators trivializing said encounters is long and not worth going deeply into. But, for example, if the strategy of a boss is "if you get a glowing glyph over your head, run over there and stand in a square formation with the other guys with glowing glyphs", an addon could use hard-to-ignore sounds and visuals to tell you that a) you have to run now, and b) exactly which corner of the square formation to run to.
I believe that in the latest raid, the developers tried restricted certain information from the API (e.g. "there is a glyph over the player's head") in an attempt to reduce that kind of strategy automation. The players then proceeded to manually feed this information (e.g. "i have a glyph over my head") into their addons using buttons mapped to API calls like this, allowing for partial automation (e.g. "your corner in the formation is top-left"). This stirred quite the drama, but the that's besides the point.
[1] https://wowpedia.fandom.com/wiki/API_C_ChatInfo.SendAddonMes...