I have struggled with this relationship and only recently have come to my own conclusions regarding it.  I would be interested to hear your opinion.

I'm not sure what Anna got out of it, other than the little house near the Pacific. She was angry with Dick at first, and rightfully so. He was the last person to see her husband alive (and arguably the one responsible for his death). But when he poured his heart out to her, she felt sorry for him. Maybe she's one of those "heart-of-gold" people who feel for hard luck cases. I think it grew into a platonic love/friendship from there.

What Dick got out of it is obvious. She was the one person in the world that knew his secret. He could be totally himself around her. She caught him in his lie, he threw himself on her mercy, and she was ok with it. That was a huge burden lifted from his shoulders.

I think they did end up loving each other, but only as good friends.

That makes sense but why arent the other women who also know his secret treated the same as Anna - so what was the significance of Anna.  I agree it was platonic but did Don see something in Anna that was similar to the discovery of a spring in an emotional desert that fed his thirst for something long gone that he never knew.  I was thinking Mother  Son.

Could be. Anna loved him unconditionally, like a mother. The others obviously didn't.