Why serve 1x1 pixel GIF (web bugs) data at all?

Question

Many analytic and tracking tools are requesting 1x1 GIF image (web bug, invisible for the user) for cross-domain event storing/processing.

Why to serve this GIF image at all? Wouldn't it be more efficient to simply return some error code such as 503 Service Temporary Unavailable or empty file?

Update: To be more clear, I'm asking why to serve GIF image data when all information required has been already sent in request headers. The GIF image itself does not return any useful information.

Answer 1

Doug's answer is pretty comprehensive; I thought I'd add in an additional note (at the OP's request, off of my comment)

Doug's answer explains why 1x1 pixel beacons are used for the purpose they are used for; I thought I'd outline a potential alternative approach, which is to use HTTP Status Code 204, No Content, for a response, and not send an image body.

204 No Content

The server has fulfilled the request but does not need to return an entity-body, and might want to return updated metainformation. The response MAY include new or updated metainformation in the form of entity-headers, which if present SHOULD be associated with the requested variant.

Basically, the server receives the request, and decides to not send a body (in this case, to not send an image). But it replies with a code to inform the agent that this was a conscious decision; basically, its just a shorter way to respond affirmatively.

From Google's Page Speed documentation:

One popular way of recording page views in an asynchronous fashion is to include a JavaScript snippet at the bottom of the target page (or as an onload event handler), that notifies a logging server when a user loads the page. The most common way of doing this is to construct a request to the server for a "beacon", and encode all the data of interest as parameters in the URL for the beacon resource. To keep the HTTP response very small, a transparent 1x1-pixel image is a good candidate for a beacon request. A slightly more optimal beacon would use an HTTP 204 response ("no content") which is marginally smaller than a 1x1 GIF.

I've never tried it, but in theory it should serve the same purpose without requiring the gif itself to be transmitted, saving you 35 bytes, in the case of Google Analytics. (In the scheme of things, unless you're Google Analytics serving many trillions of hits per day, 35 bytes is really nothing.)

You can test it with this code:

var i = new Image();  i.src = "http://httpstat.us/204";