Symbolism, Imagery, Allegory
The old symbol-o-meter should be beeping out of control when you see Lamott bring up lighthouses in the last chapter of the book. Here's what she says:
Maybe what you've written will help others, will be a small part of the solution. You don't even have to know how or in what way, but if you are writing the clearest, truest words you can find and doing the best you can to understand and communicate, this will shine on paper like its own little lighthouse. Lighthouses don't go running all over an island looking for boats to save; they just stand there shining. (29.31)
What we like about this symbol is that it implies you can help other people by writing a great story, and you can do it even without having the slightest clue how it's going to work.
Communicating about our lives as honestly as we can is, well, illuminating (pun totally intended), even if we're only thinking about our own experience at the time. Someone has probably had a similar experience, so when we write honestly, what we say may, you know, shed light (okay, we'll stop) on someone else's experience as well as our own. When we write, we may be unconsciously brightening the way for others. Who knows, if they're in a storm, we might even save a life.
What other job lets you save lives while being basically clueless about how to do it? We bet surgeons can't say that.