Think of it as a multimeter - feed in AC signal on the input, it calculates TRMS value and outputs it in terms of DC voltage (rather than LCD display). It simplifies measurement, because DC is easier to measure than changing AC signal.
If your equipment needs TRMS signal, it is useful. Most cases relate to various VFD motors and such, where AVG responding multimeters show inaccurate results. See discussions on AVG vs TRMS multimeters.
And it is not a miracle, it will not show TRMS for any random signal, just for "normal" bandwidth limited signals like sine, square, triangle, sine with small noise.