Mostly in Nigeria where I have witnessed this in different states like Enugu, IMO,Edo etc.
In Igbo land,its a culture and also the tradition across the country. When a married man dies,his wife must be in a white cloth along aside with the children if he has. During the burial,she will stay at a corner made special only for her to stay. A sit must be there and a bucket or anything that can contain money from the visitors or her In-law/friends.
They will us something to by cut her from people close call. Its only the hand can penetrate.
Some widows in Enugu state usually weren't allowed to see their dead husband, which is not good.

HERE ARE THE SEVEN REASONS WOMEN WEAR WHITE CLOTH DURING THEIR HUSBAND BURIAL
http://9jaairwaves..com.ng/2018/01/see-seven-reasons-why-women-wear-white.html