Microwave venting is there to:

Vent cooking fumes from the microwave and the range/stove below the
microwave.
Try to keep the microwave electronics form being fried from high stove
heat, like during a self cleaning cycle.
Most over the range microwaves have a temp sensor that will auto turn on
the built in vent fan.
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I cannot warm up to people who are pompous, arrogant, deceitful,
self-serving,
ill-tempered, intolerant or just plain dumb and proud of it.
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