This prefixes each line with its position in the list: 1, 2, 3 and so on.
What numbering gives you
A stable reference. Once every row has a number, people can point at row 47 rather than describing its contents, and the list can be restored to this order after being sorted.
Preserving an order you are about to lose
Numbering before sorting records the original sequence. Sort numerically on the prefix afterwards and the original order comes back exactly.
Referencing in review
Numbered lists are far easier to discuss across a team. Feedback arrives as "remove 12, 40 and 51" rather than as prose describing which entries to find.
Try it: Number Lines on SeoWolf's Notepad.