Because theme is another one of those things that keeps your story on track. It helps you decide whether you need this scene, that character, that line of dialogue; it gives you a starting point when you're stuck; it gives you something to cling to when you have to make big scary changes. It's an indicator of what your story should end up looking like, because it tells you what you'd story needs to 'say'.

IMHO Theme is the art of story-telling. Theme is/can be planned, but the most critical aspect of theme is HOW the reader/viewer connects, and creating the points of story that permit them to do so; we talk about capturing "lightning in a bottle" when what we really mean is "does the downstroke of the author's intention meet the upstroke of the viewer/reader's experience." When the two meet - lightning. Practically, this means that the writer must create the opportunity to connect with the viewer/reader; the opportunity comes via the message, the heart of the story, and answers the question "where does this connect with either the viewer's reality or something the viewer is affected by in a meaningful way". Thus, themes of "money isn't everything" or "some people want to watch the world burn" or "love conquers all". IMHO.

I think it was the great existential philosopher Homer Simpson who in "Blood Feud" [Season 2 Episode 22] best summed up the problems of a narrative presented without any thematic architecture for support: "It's just a bunch of stuff that happened."

Theme allows the writer to access and work with universal truths, notions that can connect readers and audience on multiple levels. By presenting a moral argument about a universal truth, the writer is engaging the reader in the soul of the story, the belief of the story, the parts of the story that connect it to the real world. This works to assist the reader to develop sympathy, empathy and identity with the character and his/her situation. The reader will feel.

Perhaps the most profound statement on theme that we've stumbled upon so far... :)