Yes they can be very handy.
You will have to sit down with the QS and agree the type of Spec house you are looking for, ie insulation and u values, windows, finishes spec on doors and architrave white deal vs oak. Stairs and kitchens will be a PC sum.
When you have a good spec on paper the qs will measure all work items in the house send it out to a few builders and compare prices, the BOQ is as such a contract and all rates that the builder includes should be fixed for the duration of the contract.This will give you a price certainty as to where your build is going and will allow you spend a little more or less on finishes depending on your budget. It will also allow you track any variations or extras as these will be outside the measured scope of the bill.
If you are going selfbuild like me and hoping to save a bit more money, the way it will probably work is talk to a blocklayer he will look at plans and price it per block its up to you to count them after, the same for the roof you buy the materials and pay the carpenters for their time or let them price it. None of these lads would rarely see a section from a BOQ and usually look at these jobs; ah it will take two weeks give me ah umm €5,000.
If you get a BOQ drawn up though it can be useful for ordering materials, and keeping track on how your self build sperate work elements are tracking against a price you might have recieved from a builder. If you get the bill broken out into blockwork, concrete, roofing, plumbing, plastering, wiring and second fix get a builder to price it and then use it as a way to haggle with local blocklayers roofers and so on who will be quoting you