Explain why you usually can't use just a single spectrum line to confirm the identity of an element

You can't usually just use a  single spectrum line to confirm the identity of an element because there are cases that the emission line id not clearly defined.  When the emission line is very weak compared to surrounding noise, in which case the more datapoints you have to build up confidence for the existence of a particular emission spectra, the better.

They can measure  the amount of energy gained or lost