Amity randomly casts light spells because they remind her of Luz. She doesn’t really need a light source– it just takes her back to the first interaction she had with Luz.
Like imagine after her injury she misses Luz like crazy, she is her first crush after all, a friend she chose to make on her own and wasn’t influenced by others, she is special, so she casts a lot of lights spells, just to imagine Luz is near her.
The twins are probably all weirded out each time they enter Amity’s bedroom and she is surrounded by a lot of light spells floating around everywhere. They’d be like:
Emira: Mittens, why is your room filled with lights spells all the time?
Edric: Yeah don’t we have candles that light on their own?
Emira: *rolling her eyes and looking pointedly at Edric* It’s daytime. Why would she even need candles?
And they start discussing the importance of a proper light source in order to not “damage” your eyes. Amity just ignores them (because that happens all the time). So she tunes out their voices while she continues reading. She would probably kick them out later with an abomination when things start getting steamy, but right now she’s too engrossed on re-reading Azura.