What Seems To Be The Reason Why Movie Houses Have Dim Lights?

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i just watched a movie on a big screen last week. it was a local love story movie that has a funny twist in the end. anyway, i enjoyed it even though some of my friends were laughing that it was kinda corny. as we were approaching the stairs down (we were seated at the balchony), my friend missed a step and "ouch", she accidentally twisted her ankle. OMG, i just noticed, it was dim that such incident happened. then, what seems to be the reason why they are dim lighted even though the movie was done and finished? have you ever had similar observations to movie houses near you?