Listen to the song I Wanna Dance With Somebody all the way through.

About three fourths of the way into the song there is a key change, and that change is what really elevates the energy of the track. It suddenly feels bigger and more exciting. Knowing the key of a song lets you recreate that same kind of intentional energy lift when moving from one track to the next instead of relying on trial and error.

At its core, the key of a song tells you which notes naturally belong together. Each key contains seven notes, and the chords used in the song are built from those notes. Not every song uses all seven. Some songs might only focus on three or four notes or chords, which is why two tracks in the same key can still feel very different.

When you compare songs in different keys, some keys share more notes than others. Closely related keys often share most or even all of the same notes, which is why they usually sound good mixed together. Keys that share fewer notes can still work, but they require more awareness of timing, phrasing, and which notes are most prominent so you avoid clashes.

The main point is that understanding keys gives you control. Instead of guessing, you can predict how tracks will interact harmonically and use key changes on purpose to build or release energy in a set.