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Replacement of the loaves was essential because the Torah commands that the loaves be placed before God tamid – “always” – (see Ex 25:30).
Rabbi Yosei disagrees with this understand of tamid, arguing that even if the loaves were removed in the morning and replaced in the evening, it would still fulfill the requirement of tamid, which should be understood as teaching that a night should not pass without Shewbread on the table.
Rabbi Ami infers from Rabbi Yosei’s teaching that the concept of tamid – “always” – need not be understood as requiring 24 hour vigilance, and applies it to other situations, as well.
Specifically, he says that Torah study, which is a requirement “day and night” (see Yehoshua 1:8) can be fulfilled by studying a chapter in the morning and a chapter at night.
Reish Lakish teaches that one who forgets even one thing from his learning transgresses the Torah prohibition of, "Rak Hishamer Lecha... Pen Tishkach Es ha'Devarim" -- "Guard yourself... lest you forget these things" (Devarim 4:9).
The Gemara clearly understands that this verse refers to forgetting Torah. However, the straightforward meaning of the verse is a reference to the giving of the Torah at Har Sinai, when Hashem commanded the Jewish people not to forget the things they saw there.