If you think of each row as an equation perhaps concerning the cost of some items, then multiplying by a fixed number doesn't tell you anything new:
(2 quarts of milk + 3 bagels cost $3.50) x 2
4 quarts + 6 bagels cost $7.00
But when you combine multiplying a row and adding (or subtracting) from another independent row, then you can work out the cost of the individual items:
2 quarts + 3 bagels cost $3.50
7 quarts + 4 bagels cost $9.00
Now you can work out that a quart costs a dollar, and the bagels are fifty cents each: to do it formally you might multiply the first row by 4/3 and then subtract from the second row to eliminate the bagels from the result.