What are the reasons for using layers?

Other than spamming what are the principal reasons to have layers in a page?

Thanks in advance!

Honestly, layers were great back in "the day". They allow a person to place material anywhere on a page based on the X and Y axis.

Where it becomes relatively useless is when the screen resolution of the target audience is different then what you have written for. Misaligned text/images/flash, etc will apear in different locations depending on screen resolution, and/or browsers.

A fix has been created using stylsheets with the absolute/relative positions.

As for a description on layers, think of a smoothe pond of water as the web page. Now place a little paper boat on the waters surface, anywhere you like as a layer.

Hope this adds some insight.

Cheers

Layers have many dhtml uses. They can provide animation, replace framesets by using z index, they can provide text readers for the blind with descriptions, they help with positioning a graphic exactly in tables where the img tag wont look correct (margin widths).

Css layers have really quite a wide variety of uses. Many I'm sure I havent come across.

Google is not going to penalise pages with css layers if that is what you are getting at. There are simply too many legitimate reasons for them.