Mr.__ is a widower with children who needs a wife to run his household. He wants to marry Celie's pretty younger sister, Nettie. However, their father has his eyes on Nettie for himself and refuses to allow Mr.__ to marry her.
Celie dislikes Mr.__ and doesn't want to take care of someone else's children, but she notices the way her father is looking at Nettie and wants to save her sister. Therefore, she starts proposing herself as the appropriate wife for Mr.__.
This seems startling, given Celie's aversion to the man, but her reasons become clear as she writes:
All I thought about was Nettie. How she could come to me if I marry him and he be so love struck with her I could figure out a way for us to run away.
Celie thinks that by marrying Mr.__, she can save Nellie from her father and that the two of them can live in Mr.__'s household and find a way to escape. Celie sees her marriage to this man as a temporary measure that will allow both she and Nellie to find freedom.
This shows us that Celie has a generous nature and a willingness to sacrifice herself, at least temporarily, to help someone she loves. However, even if only subconsciously, Celie is likely influenced by the photo of Mr.__'s lover she sees, the glamorous Shug Avery. Marrying Mr.__ would bring her closer to this woman.