Why do eggs have expiry dates?

All of the answers you are looking for have been touched on in various parts of this question: How long can I keep eggs in the refrigerator?

Summary:

• Expiration dates are for best quality only. Nothing magical happens on that date.
• Truly fresh eggs (i.e. from the chicken) will hardly degrade at all even after a year in storage.
• Supermarket eggs are washed, removing the protective layer and lowering the shelf life.
• Some supermarket eggs are already contaminated with salmonella, and there's no test you can do to find out for sure (other than eating them raw). This will cause serious problems if you leave them in storage for several months, unless you freeze them.
• Lastly, damaged eggs will simply go rotten. You won't always see the damage, but you'll definitely smell it by the time you hit that best-before date.

As with just about every other food, you're better off interpreting the sell-by or best-before dates as a conservative guideline; any food can go off before the date if it's been damaged or mis-handled, and many foods will still be fine long after they "expire". StillTasty.com is a good resource for finding out how long various foods actually last in storage.

The sell by date isn't so much going bad (bad eggs are rather obvious) but Salmonella.

Most eggs, especially factory farmed, have traces of salmonella on them - it can pass through the shell. But unless you are already ill you can handle it, the older the are the longer the bug has add to get into the egg and grow.