In general, when a person davens for himself, he may be told, “Who are you to even daven before Hashem?” Every time a person davens, he needs siyatta diShmaya to chase away the מקטרגים. We see this from the fact that every time we daven, we invest so much avodah into the section of דזמרה פסוקי in order to cut away the מקטרגים.
The Beis Aharon of Karlin teaches similarly (Parashas Chayei Sarah) that we repeat the pesukim of the ketores after davening is to once again chase away the מקטרגים—for it is after davening that we receive the shefah, and we need extra protection so that they shouldn’t be able to steal it. Explains the Tiferes Shlomo: When a person davens for his fellow, even if he himself is unworthy, his tefillah will nevertheless be accepted—for Hashem says ארחם; אשר את ורחמתי אחן אשר את וחנתי when you’re davening for someone else, even if both of you are unworthy, the tefillos will be accepted (and now we will explain why).
