That's kind of the point; instead sticking your finger in the air and estimating rough numbers like "4 to 5 sqm per bag", you can get fairly precise numbers very quickly, while still allowing for a little waste.
I did a comparison, where I estimated from experience how much of what I needed for a 22m2 bathroom. My eye-estimate was materials for around £590 (bear in mind that things are more expensive where I live, and the regs are different). When I entered the specs of the room into the spreadsheet, the result was £530. That's a total differance of £60 (£3 per square metre).
That's enough to lose you the job when quoting.
You'll also avoid buying too little of something.
Edit: Indeed I have, Grumpy
