Question:
By the time we get to the evening of 14 Nissan our houses are pretty much spotless and there is no chametz anywhere. We even have to lay out bread to make sure we are saying the bedikat chametz bracha for a reason (not levatala).
I understand that Torah law would just require us to to nullify our bread but the Rabbis enacted that we need to do both nullification and burning. This is because your nullification may not have been wholehearted or you may have missed some and may come to eat it. I also understand there is a separate mitzva of Tashibitu (to destroy your chametz).
All of that is assuming you still have chametz lying around that you actually own:
1. If you know for sure there is no chametz in your house and
2. You are going to sell it anyways (there is no issur of walking past seeing chametz owned by a non-jew.)
Why do we need do bedikat chametz and biur chametz before chag? (I can understand that if you find some lying around on chag then you must burn it)
Answer:
The chometz that we sell to a non Jew has to be locked away and out of sight with a mechitza so we will not come to eat it or use it. Besides this, we have a mitzvah to look and get rid of chometz that is in our possession on the 14th, and in truth we should really leave somewhere in the house that we have to check. This is also one of the reason that we put the 10 pieces out.
