It's often a more useful way of describing benefits than by forcing them into some mould of quantification based around the false assumption that everything can validly be rated in units of pounds and pence, that produces results with the spurious authority conferred by numerical precision while concealing their fundamental uselessness deriving from basically having been pulled out of someone's arse.
This is a large part of why I despise the modern obsession with "BCR" assessments and the accompanying attitude that "report X gives a BCR of Y" is a cast-iron argument for or against whatever the report is about. A large proportion of the "costs", and well-nigh all of the "benefits", are either just plain made up or are equally meaningless numbers attempting to describe the colour of the sky in terms of money. Since the whole thing is 90% fudge to begin with, not only is it irredeemably meaningless anyway, but it is also trivial to make the conclusion support whatever outcome the people commissioning the report want it to support, and it would make far more sense to just not bother with the report at all.
People seem to have got the idea that because there are essentially no railway projects that don't attract the argument against them that you'll never even come close to getting your money back, the appropriate way to counter that argument is to make up amounts of money and assign them to various useful outcomes of the project to derive a figure that argues that actually you do get your money back. Of course what then happens is that other people can also make up amounts of money and assign them to other aspects of the project to argue the other way. The end result is that much time and money is wasted on making up figures and giving them to graphic designers to make fancy PDFs with, which not only doesn't get things any forrarder, but in practical terms moves them backwards - further discussion is hampered by the need to convert worshippers of the meaningless figures, and during the time wasted messing about the non-made-up costs have increased so much you pretty well end up starting all over again.
A more appropriate counter would be to point out that nearly everything cannot be meaningfully described in terms of money, therefore attempting to evaluate any project the overwhelming majority of whose consequences cannot be so described - which includes all railway projects, and indeed nearly all projects apart from the internal machinations of a financial institution - cannot be valid and it makes more sense to consider how useful it is.
This is especially the case when considering two projects that are interrelated - such as "providing a through route Plymouth-Tavistock-Okehampton-Exeter" and "providing a diversionary route avoiding Dawlish". The way the money approach is carried out means it pretty well has to consider them as purely isolated projects and ignore the interrelation, whereas with the usefulness approach the merits of a solution that satisfies both provisions with a single construction cannot be sensibly ignored.
