Who uses graph equations?

Answer 1

Well I'm a design engineer and I use them all the time.

If I know a few values that are true for say, power versus cost for an car engine, then I can make sensible estimates for the cost of more powerful car engine that hasn't been built yet. Equally I could use graph equations to get the answers for the power I could expect from a cheaper engine.

That's obviously just one of millions of possible applications to graph equations. You use them in Engineering, Design, Accountancy, Business, IT, Marketing, Banking, in Medicine, and in daily life for anything if you know how - take an iPhone for example, it works out how much battery it has left using a graph equation. An engineer built that into it.