Because 'going through the whole thing' is nearly impossible for some people, who either log on spargingly or have a lot of friends. Additionally, since Faceboo

Because 'going through the whole thing' is nearly impossible for some people, who either log on spargingly or have a lot of friends. Additionally, since Facebook is full of relationships that are less than actual 'friends', a lot of people have complained for years about the quality of the posts. I think they must have realized their algorithms needed a stronger signal than what they had, and this UI is a way to get that data.