Pesah - Haggada
It is interesting to note that on erev Pesah, after we have spent much time and energy cleaning and removing the hametz from our homes, then we again searched for more "just in case", and we then annulled anything we might have missed, and we then burned whatever we found, we still make yet another announcement annulling any unfound hametz.
Hametz, we are told by the meforshim, represents our actions of sin. Pesah is a time where we remove the hametz from the house but also from within us. We improve our ways and repent from the sins we have done.
So, we clean our house and get rid of any hametz. We also clean our actions and get rid of any bad traits and habits.
Then we search for for still anything we might have missed.
Then we still say the oral annulment in case anything was missed.
We go through a lot of hoops to in the goal of self-improvement.
This announcement of Kol Hamira is said commonly at the time the korban pesah was being offered in the mikdash. As we get closer to Pesah, as we get closer to the time of closeness with Hashem, and as we bring the korban pesah, we should be trying harder and harder to remove the hametz - any trace of impurity in our actions and thoughts. That is why even after we have done everything we can to get rid of the slightest traces of hametz, we still have to make this announcement of annulment. At this time when we are trying to get so close to Hashem we have to go the extra mile.