Only() - although it appears to be an extremely straightforward function - is actually a very advanced aggregation function that will save your day more than once. If the use of Only() isn't apparent to you yet, don't worry. You do not need it. You have a solution but no corresponding problem, so just ignore it for the time being.
One of my preferred uses for Only() is whenever I need to apply set analysis on very few values to select a single one. Only() is the only function that let's you pick a single value from a range by simply using set analysis.