What your pay is genuinely worth once inflation is accounted for. A raise that lags inflation is a pay cut, and the only way to see that is to compare in real terms.
The real wage calculator converts a salary between years using actual inflation data, so you can see whether a raise kept pace. It works in both directions: what an old salary is worth in today money, and what you would need to earn now to match a past standard of living.
It also solves the question people usually want answered but rarely phrase directly: what figure do you need to ask for, to genuinely be ahead rather than standing still?