In the name of Allah, We praise Him, seek His help and ask for His forgiveness. Whoever Allah guides none can misguide, and whoever He allows to fall astray, none can guide them aright. We bear witness that there is no one (no idol, no person, no grave, no prophet, no imam, no dai, nobody!) worthy of worship but Allah Alone, and we bear witness that Muhammad(saws) is His slave-servant and the seal of His Messengers.
Allah says in the Holy Quran Chapter 2 Surah Baqarah verse 234:
If those of you, who die, leave wives behind; they should abstain (from marriage) for four months and ten days. Then, when their waiting term expires, they are free to do whatever they choose for themselves, provided that it is decent. You shall not be answerable for this. Allah is fully aware of what all of you do.
Thus the reason a woman, who truly believes in Allah and the Last Day, must sit in 'iddat' for the prescribed period because it is the command of Allah Subhanah. And we as believers must believe that each and every command of Allah is based on His Perfect Knowledge and Wisdom.
We are also allowed to search and understand the wisdom, so that we may be able to obey every command of our Lord and Creator wholeheartedly. The main reasons for this waiting period are:
Whatever written of Truth and benefit is only due to Allah's Assistance and Guidance, and whatever of error is of me. Allah Alone Knows Best and He is the Only Source of Strength.
Your brother and well wisher in Islam,