Quick! What does the formula "=sum(A5:A15) * $B$2" mean?
Of course, you have no idea. Sure, it sums up some numbers and then multiplies the result by something else, but we have no idea what it's really up to. Even if you were looking at it in context (i.e., with the rest of the spreadsheet) it might be confusing.
OK. How about "=sum(car_sales) * sales_tax_rate"?
Obviously, this one makes more sense. We're computing the total sales tax on a group of automobile sales.