In The Color Purple, to whom does Celie write most of her letters? Why?

That answeris not correct.

Celie DOES write her letters to God initially, but when she discovers the letters that Mr. ____ (or Albert) have been hiding from her (from Nettie), she starts to write to Nettie, not knowing if Nettie receives them.

Nettie and Albert were not dating at any time. Albert always wanted to marry Nettie but she was too young, and Albert was forced to pick Celie. After Celie moves out, Nettie moves in with Celie and Albert. When she realized she couldn't stay with her sister and Albert, she joins Corrine and Samuel (she is not forced) on a missionary trip to Africa.

She writes to God because she is lonely and has no one to talk to. She writes to Nettie after God because she has hope that Nettie is alive and well.