Why do flys like to fly around the light fittings in our homes?

i noticed with flys they are attracted the the light fittings in our homes, why?
Update: cmon you can do better than thats what flies do? minus 10 for that poor excuse of an answer. i want more informative answers.
Update 2: i turned off the light and they still fly round and round, a plus point however is i know where they all are when i get the fly killer lol.
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