Question

why is it helpful to write numbers vertically when you use the subtraction algorithm?

Explanation:

Align place values

Writing numbers vertically aligns digits by place value.
This keeps ones, tens, and hundreds organized.
Using Column Subtraction with Regrouping requires this alignment.

Simplify regrouping

Vertical alignment makes regrouping much easier to track.
You can clearly write borrowed values above.
This prevents calculation errors during multi-digit subtraction.

Answer:

Writing numbers vertically aligns the digits by their place values (ones, tens, hundreds, etc.), making it easy to subtract column by column and to perform regrouping (borrowing) accurately.