[sanswork]

It's not just for shortening urls it's also for deciding where your search engine scores go. So to take an example from a googler at a recent conference say you have an online store that has a bunch of categories. Someone wants to buy a red bag and they go through bags then to red and pick item 5. /bags/red/5

Someone else goes to red things then bags than item 5 /red/bags/5

You end up with duplicate content issues and the possibility of having your search engine score split between the two pages.

With canonical you can say on both pages that /items/5 is the real url and thats the one that will get the search score + be indexed most likely.

[sachinag]

All you have to do is set up a rule to have the screening terms in a certain order. We did on Dawdle in like a day when we realized our in-stock item screening presented the issue: http://www.dawdle.com/search.php/

I was pretty convinced that canonical was a solution in search of a problem, but at least joshu's opening salvo, the DiggBar brouhaha, and the resultant conversation has shown one use for it.

[gregoryl]

It leads me to wonder, if google is as smart as most people see them as, why don't they make an attempt at working this out, or at least defining a way to deal with it themselves?

[sanswork]

They kind of do but this gives site owners a method to instruct Google on what they want stored about their sites. You can't figure that out with an algorithm.