Productivity != Profitability
IT staff don't get paid what they're worth to the company - that's almost certainly true. The sad fact is that most of us get PAID FAR MORE than the profit they bring in would support. Sure, there are other requirements: such as meeting legal/financial obligations, but for most people in IT - whether programmers, testers, designers, support people, project manglers, QA-ers, planners, or trainees there is no direct connection between what they do and their employers' income. You cannot point to a line of code and say "I wrote that, and it earns us £1,000 a year."
At best, IT people can say "without us, the business would be much less efficient and have to employ many more staff, to do things manually." However that's full of intangibles, suppositories, and guesswork. Luckily no CEO has ever challenged that theory (like no CEO ever has the cojones to go into the datacentre and press the BIG RED BUTTON to see of the D.R. plan actually works).
On the flipside, this does mean that an IT-er should be able to write a reasonably credible account of themselves. Since nothing is tangible, accountable or verifiable you can easily say "I earned the company £X,000 last year (where "X" should be greater than your salary and expenses, employers NICs and office rental). and no-one will be able to challenge it. It could fall apart if some sharp-eyed HR person spotted that the entire justifications of the IT team came to more than the company earned - but that'll never happen: they're all too busy trying to justify their own, even more tenous raison d'etre.