Bismillah,

All praise is due to Allah, and may exaltation and safety be granted to Muhammad, his family, his companions and those who aid them. To proceed:

Al-‘Allaamah, Saalih al-Fawzaan, may Allah preserve him, said:

…The wisdoms behind the legislation of the waiting period (al-‘Iddah) (1) can be summed up in the following points:

[1]: Ensuring that the woman’s womb is clear from pregnancy, so that another man does not [marry her and] have intercourse with her before knowing whether she is pregnant or not, and as a result, lineage becomes uncertain and lost;

[2]: Magnifying [the worth] of a marriage contract;

[3]: Prolonging the chance [for the husband] to reconsider staying with his wife;

[4]: [If the woman’s husband has passed way], the waiting period gives the deceased his rights. And by preventing the woman from beautifying herself (2), [in the case of her husband's death], it displays the significance of his loss… (3)