Billy's relationship with the speaker

Slaughterhouse 5 by Kurt Vonnegut

The relationship between Billy and the speaker. Is as confusing and obscured as the events of this book. For the longest time, I even assumed that the speaker had once again changed his name for the book as he said he did in the beginning. Among many things, the speaker and Billy do have some very oddly peculiar moments. Moments that are so short and so insignificant yet as similar as can be. One example is when speaking of their dogs. Back in the beginning of the book, the speaker states, "I let him know I like him, and he lets me know he likes me" (Vonnegut 7). Much later in the book, Billy says something eerily familiar to this same sentence by the speaker. Also the speaker and Billy both share an aurora of lunacy to them. The speaker has his completely irregular and concise sentences, and Billy has the whole being abducted by aliens and time travel idea. However, later on through some very sneaky and inconspicuous hints, one realizes that Billy and the speaker are in no way the same person, considering the speaker exists within Billy's story. Intriguingly their paths cross, and some of the character's, such as  Paul Lazzaro and OHare are all present within Billy's version. This possess the question, if the speaker was present through most of Billy's war experience, why did he tell the entire story through through Billy's version Then again, why did Kurt Vonnegut himself even include the initial speaker in his book and why didn't he just have the story told solely from Billy's first person perspective? I guess we may or may not know soon enough.