The scenario is that you (like me, back in the 70s!) learned all about Calculus (mainly the eponymous Derivatives and Integrals) at school or college - they're tremendously useful at applying maths theory to the physical world, a world where properties change over time, where everything's constantly decaying or evolving.
Fundamental to calculus are remembering some of the main formulae and results - even hardened mathematicians don't have to derive each from first principles each time! You might choose to try and commit all the main ones to memory or you might carry around a laminated card with the ones you need. Or you might install Derivatives and Integrals on your Symbian smartphone.
Derivatives and Integrals is fine as far as it goes and may indeed be a useful set of calculus reminders for scientists and engineers. In truth, at only £1 in the Nokia Store it's not a huge purchase and a price that's not worth worrying about. Having said that, it doesn't currently do much that a few well crafted static web pages or a Word file couldn't do - I'd have liked to have seen some extras included: links to recommended calculus web sites, numerical analysis sites or modules, that sort of thing.