Before we arrange any song for our worship team, the first question we always ask is: what key is this in? It sounds basic, but knowing the key unlocks everything else — which chords are "at home" in the song, where the natural tension points are, and which key might suit our vocalists better if we need to transpose.
We use this tool when we get a chord chart that doesn't mention the key, or when we've transposed a song several times and lost track of what key we're actually in. Enter the main chords — even just the verse chords are enough — and the key identifier does the rest. It's particularly helpful for newer musicians on our team who are still building their music theory knowledge.
Pro tip: if the key finder suggests two possible keys, the one that contains the chord you feel like the song "rests on" at the end of a phrase is almost always correct. That resting chord is called the tonic — it's home base.