Most bills of quantities do one job and then stop. They are prepared, they are used to get prices, a contractor is chosen, and the document goes into a folder nobody opens again until there is an argument.
That is a waste of the most useful thing on the project.
What it actually is
A bill of quantities is a measured list of everything the works require: so much excavation, so many blocks, so much reinforcement, so much roofing, priced per unit. Two numbers per line: how much, and what each costs.
The pricing half is what everybody uses. The measurement half is what makes cost control possible, because it is the only place that says what the build should consume before it consumes it.
Why that matters mid-build
Without a measured expectation, the question "are we over budget?" can only be answered by adding up what has been spent, which tells you where you are but not whether that is where you should be. Half the money gone at half the build is meaningless on its own: it depends entirely on which half.
With one, the question becomes answerable weekly. This element was measured at this quantity; we have consumed that much; we are that far through it. Now spending has a denominator, and an overrun announces itself in week six rather than at handover.
How to make yours useful
Keep it in a form you can compare against. A priced PDF from the quantity surveyor is a record. The same figures in a sheet, element by element, is a control. Whatever you use has to let you put actual next to planned.
Record deliveries against elements, not just in total. Cement received is a number. Cement received against the blockwork element is information. See why materials go missing on site for the receiving discipline this depends on.
Update it when the design changes. A variation that is agreed but never reflected in the bill turns your baseline into fiction, and every comparison after that point is meaningless.
Look at the variances, not the totals. Three elements will be badly out and the rest will be close. Find the three.
The most common failure
Not that the bill is wrong. It is usually a reasonable measurement of a reasonable design. It is that nobody compares anything to it until the project is finished, at which point it becomes a document for arguing about rather than a tool for deciding.
The habit worth building is small: once a week, take the elements currently in progress and put actual next to planned. Ten minutes. It is the entire difference between knowing and hoping.
This is what Build is designed around: a bill of quantities that stays live, with goods-received notes and labour posting against elements, and budget-versus-actual updating as the site spends. The weekly comparison works on paper too, though, and it is worth starting on your current project rather than your next one.