This isn’t a question about why it’s there. What I want to know is, why aren’t we required to do anything with it during the seder? We don’t eat the lamb bone, but at least it gets a mention.
I asked my brother the same question and the only thing we could come up with was that it was a “generic fertility symbol” which doesn’t really make a lot of sense given the context…
The egg is in rememberence of the Korban Chagigah (the Festival Offering).
Jews in Israel were required to go to Jerusalem three times a year - on Passover, Shavous and Succos. At that time, they were required to bring a sacrifice, called the Korban Chagigah. It is in commemoration of this that the egg is on the seder plate.
One is not requried to do anything with it after the seder. It can be eaten or discarded.
Zev Steinhardt
Horseradish. It’s not just for Seder anymore. ;j
That explains why it’s there. I’m wondering why there’s no place in the seder where you’re required to actually do anything with it, or even acknowledge that it’s there. Did the addition of the egg occur after the finalization of the 15 steps of the Haggadah?
I don’t have an authoritative answer for this, but I can give you my gut feeling on it.
Most of the items that we mention at the Seder are those that deal exclusively with Passover and the Exodus – the matzah, the marror, the Passover Sacrifice. The Korban Chaggigah was not exclusive to Passover, but yet was present at every Seder (in Temple times). As such, when the Passover sacrifice was “replaced” with the shankbone after the Temple’s destruction, the Chaggigah was replaced as well. But since the Korban Chaggigah had no special place at the seder in Temple times (other than the fact that it was eaten as part of the meal), it likewise retained no formal role after the Temple’s destruction.
Zev Steinhardt
When I was a kid, our dog grabbed the lamb bone and ran off with it before anyone could stop him. Later, upon close examination, we discovered that he wasn’t even Jewish. 
Why not? Passover is also the Festival of Spring. Jewish holidays can wear several “hats,” so to speak, and that’s one of them.