In those constructions, "as" suggests that the two observations are actually related. "When""”which can still be used in such a situation"”does not necessarily carry such an implication.E.g.,
The traffic starts to get heavier at 3 p.m. as students get out of school.
"”> This sentence implies that the students contribute significantly to the traffic.The traffic starts to get heavier at 3 p.m., when students get out of school.
"”> No such implication; just two observations that happen to occur at the same time. (The point of this sentence could be something like, "Poor students... just when they get out and want to drive somewhere, there's traffic.")