In the novel To Kill a Mockingbird why did Boo Radley put soap dolls in the tree?

Boo Radley left the soap dolls in the tree for Scout and Jem to find in order to reach out to them in friendship.

Boo Radley was the neighborhood recluse.  All of the children were afraid of him, and all of the adults pitied him.  Scout, Jem, and Dill were fascinated with him, and it was Dill who came up with the idea of trying make him come out.  The children engaged in many different plans over the summer to accomplish this.  They never had much direct contact with him, but they made an impression.

Boo Radley obviously began to feel a connection with the children.  He demonstrated it to them by leaving them presents in the knothole of a tree on the corner of his property.  The children did not know who the gifts were for, or what the point of leaving them there was, until they found the soap dolls.

I pulled out two small images carved in soap. One was the figure of a boy, the other wore a crude dress. Before I remembered that there was no such thing as hoo-dooing, I shrieked and threw them down. (Ch. 7)

Pennies, twine, and gum could have been there for any reason.  However, the soap dolls look specifically like Scout and Jem.  This is why Scout is frightened by them.  She thinks they are something like voodoo dolls, designed to hurt them.  Although Boo did not intend to frighten the children, making them look like Scout and Jem was a clear sign he wanted them to know the gifts were intended for them.

It took a great amount of skill to make the carvings.  Jem points out that they are the best he’s ever seen.  By carving the children’s likeness in soap and giving it to them, Boo is trying to reach out and communicate with the children in the only way he knows how.  He is too shy to talk to them directly.  He is showing them he likes them as friends.

As Scout gets older, she also gets wiser and more empathetic.  She comes to see things from Boo’s point of view rather than be afraid of him.

I sometimes felt a twinge of remorse … at ever having taken part in what must have been sheer torment to Arthur Radley—what reasonable recluse wants children peeping through his shutters, delivering greetings on the end of a fishingpole, wandering in his collards at night? And yet I remembered. Two Indian-head pennies, chewing gum, soap dolls, a rusty medal, a broken watch and chain. (Ch. 26)

At the end of the book, Boo Radley does come out—when Scout and Jem’s lives are threatened.  He saves them by killing Bob Ewell, and then takes them home.  Scout realizes that he is a sensitive and timid man, and also a good friend.

Boo was our neighbor. He gave us two soap dolls, a broken watch and chain, a pair of good-luck pennies, and our lives. … We never put back into the tree what we took out of it: we had given him nothing, and it made me sad. (Ch. 31)

Standing on the Radley porch, Scout sees things from Boo’s point of view.  She reviews the events of her childhood from his perspective.  She knows that he cares about them, and she cares about him.

The interactions with Boo Radley demonstrate Scout and Jem’s transition from child to young adult.  They go from viewing Boo as a boogey man to realizing that he is a sad, lonely man who just wants a friend.  Since none of the adults in Maycomb understand him, he reaches out to children who show him kindness.