It does a few things but it basically entrains your circadian rhythms that mediate wakefulness and sleep. I’m no scientist but they are helpful for both insomnia and depression (particularly SAD) and I find it to be an indispensible part of my morning routine in waking me up and orienting me and fixing my sleep to a proper schedule.
And the light is important in the winter anyway because of the dreariness and darkness but I can’t justify why