When does one experience his failings in kedushah most acutely? What part of one's avodas Hashem is most tangibly affected by a breach in kedushah?
It is undoubtedly his davening.
Why?
Because, as R' Chaim Soloveitchik tells us, the very definition of praying is omed lifnei haMakom, the feeling of standing before Hashem and engaging in a dialogue with Him. Ramchal writes beatifully that tefillah is like a person speaking to a friend. In fact, R'Moshe Shmuel Shapiro, the late Rosh Yeshivah of Be'er Yaakov, would sing these words on the way to shul each morning.
The improper images that have been seen by one who has not guarded his eyes are in the forefront of his imagination. When he then wants to daven, when he would like to visualize that he is speaking directly to Hashem, those other images come to the fore and make him feel like a fraud. "How can I face Hashem and talk to Him when I am ashamed of what I just saw?" He feels like a guilty child caught with his hand in the cookie jar, who cannot bear to directly face the parent and instead stares at the floor.
This is true not only for children but even for adults. A polygraph )lie detector( works on this very premise. When someone feels like a fraud, he experiences a conflict between that which he knows within himself to be true and that which his mouth is saying on the outside. He is embarrassed, and the lie detector sens s the various physical manifestations - the quickening of the pulse etc. - of that conflict.The trained expert can read the polygraph results and establish that the person is lying.
Someone who does not guard his eyes cannot, metaphorically, "look Hashem in the eye," and his tefillah suffers accordingly.
On the other hand, one who guards his eyes is proud of the relationship he has with his Maker, and can talk to Him directly and honestly.
Indeed, Harav Yitzchak Zilberstein writes that he once heard the following from the Shomer Emunim Rebbe. Chazal say that one who sees an improper sight and turns away without allowing himself to enjoy it merits to see the Shechinah. Since at that time, the Shechinah rests upon him, it is a propitious time to ask Hashem for all his needs. While his eyes are looking down and he is suppressing his desire to look, Hashem will surely answer his prayers.