Our Gemara on Amud Aleph teaches us that the incense achieves atonement for L’shon Hara, slander. Let something that is done in secret, i.e., the incense, which is burned in seclusion within the Sanctuary, come and effect atonement for an act done in secret, i.e., slander, which is generally said in private.
Ramah in Toras HaOlah III:7 says a number of chiddushim in regard to this Gemara:
The incense is brought in seclusion, meaning that no one else is permitted to be in the temple when the cohen is bringing the incense. The reason for this is because it is a deep meditative service, (as we have described elsewhere see psychology of the Daf, Yoma 25), it is best performed without the distractions of anybody else around. The Ramah goes on to say something surprising. The incense is not really to atone for L’shon Hora literally. This is because we already have a teaching that the cloak of the Cohen Godol is to atone for L’shon Horah. The cloak’s trilling bells, which mimic the sounds of gossipers, are a reminder of that.
The Ketores here is atoning for a different “wrong speech”. It is atonement for thinking improperly and/or revealing private and mystical secrets about God which are appropriately kept private. Though the deep mystical secrets are part of the Torah, it is forbidden to teach them publicly, nor to teach them to an unworthy student (Chaggigah 11b). Even when teaching them to a worthy student, it only can be in a one on one interaction and via indirect hints. Thus, the private and intimate meditative practice of the ketores occurs with nobody else inside the sanctuary as a reinforcement, an atonement if you will, of improper mystical revelations and thought processes.
Translations Courtesy of Sefaria, except when, sometimes, I disagree with the translation 
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