Suppose cells in a column (TheSheet!A) contain just references to cells in the column of another sheet, but noncontiguously. For example, give Sheet!A17 the formula:
=OtherSheet!A23Other cells on TheSheet in the same row correspondingly need to reference cells in the referenced row. For example, TheSheet!B17 would need to reference OtherSheet!D23. While the references could be written directly, it would be error-prone. How can these other cells instead use the cell references from column A in their own formulae?
Note the value of the cell isn't a cell address, so INDIRECT is insufficient. Similarly, ROW by itself is unsuitable as it will return the row of the cell itself, not the cell it references.