I don't see how Sati could clear the sky in the real world, unless she somehow figures out a way to control the little things up in the atmosphere that are blocking the sunlight. 

Maybe she was 'that which matters most' because she was an AI conceived out of love, not some predetermined purpose.  Or maybe it was because through meeting her, Neo learned that at least some Machines could love...which might have led to him coming up with a solution that saved both Zion and the Machines.

 

Illyria

Neo had implants from the Machines in his body, the same as every other redpill, but he was also the One.  The program in him that made him the One allowed him to send a command to the sentinels he stopped.  Sati would need some kind of link to the real world that would let her interact with what had blocked the sky.

Illyria