That Atticus has Scout and Boo move out to the porch rather than in the living room in Chapter 30 of To Kill a Mockingbird, is evident, certainly, for the reasons already defined. However, knowing also what a timid character Boo Radley is, as well as the fact that he has not been around anyone other than his own parents in years, may enter into the decision of Atticus. For, Boo is almost like an inmate who has been in solitary confinement, and being seated in a room with four walls and in the midst of others may, perhaps, be traumatic to such a man.
By being out on the porch, Boo does not feel confined, and he has both the opportunity of walking away if he wants as well as the opportunity of viewing surroundings familiar to him: his house and the street that he watches the children play and walk near. Thus, Atticus's action to move Boo onto the porch underscores one of the themes of Harper Lee's novel that Scout reiterates at the end of the chapter:
Atticus was right. One time he said you never really know a man until you stand in his shoes and walk around in them.
Boo has been kept "in the dark" both literally and figuratively for a long time. This has been done largely for his own safety (having seen what this town does to those who are different and having hear the wild tales about Boo, it should be evident by now that he is nowhere near the threat Maycomb residents and lore make him out to be). No matter what the reason, there is no need to "expose" Boo now, especially after he has saved the children. Scout begins to realize this and starts to understand that Boo is harmless bur also afraid - a position she has been in herself. Atticus knows that the safest place for Boo is the shadows as well n(look what happened to Tom Robinson) - just because it's a sin to kill a mockingbird does not mean that Maycomb is a town free of sinners!
Atticus suggests that they go out on the porch in Chapter 30. Scout does not understand why Atticus wants to do that, but then she understands. She realizes that the lights in the living room are very bright. This is why Atticus wants Boo out on the porch.
The reason for this can be seen at the end of the previous chapter (Chapter 29). Boo Radley is described there and Scout really emphasizes the fact that he has not seen the sun in years and really looks like it. So the idea is that Boo is not used to bright lights and might be more comfortable in a dimmer place.