"Please switch off the air conditioner."
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If you put your mouse over the word 了, you see that it is an interjection in this case. Wikipedia says that "in linguistics, an interjection is a word or expression that occurs as an utterance on its own and expresses a spontaneous feeling or reaction"
Meanwhile, it is weird to say it without the 了 because the sentence would feel incomplete. Instead of using 关了, you use 关 and a preposition e.g. 关掉 or 关上 to have a complete verb
I think in this case, 了 is expressing the "new situation" of the air conditioner being off (that will be a new situation, because the air conditioner is on now.) However, you need to put something where the 了 is in this sentence. When you use the 把 sentence structure, you can't end it with a single character verb. There has to be some kind of complement after the verb. So 了 is filling a grammatical need here.