Reports for people often show a label once and leave the rows below it empty: North, then two stores, then South. For analysis every row needs its region. Transform > Fill > Down copies each value into the empty cells below it, until the next value.
Fill Down only fills null. Empty cells in an Excel file arrive as null, but empty cells in a CSV file arrive as empty text, so Fill Down seems to do nothing. Replace the empty text with null first (Replace Values: find nothing, replace with null), then fill.
Add Column > Index Column numbers the rows (from 0 or from 1), and Keep Rows > Keep Top Rows keeps the first N, which after a sort means the top N.