There really isn't any PRACTICAL difference. Grammatically though, "Be here AT 8:00am" means "I don't care where you are at one second before 8:00am, nor do I care where you are at one second past 8:00am, but on the dot of 8:00am, be here". Whereas, "Be here BY 8:00am" means "arrive at some point in time before 8:00am, and remain here until at least 8:00am".
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