If a story is a train track, a subplot is the trestle. The main plot takes you where you need to go, from point A to point B and all destinations in between. But without the subplots, it would never make it over the valleys that inevitably manifest during a story arc.
Some people define story as characters acting within a setting. I’ve never totally bought in to that, but I do believe that characters generate subplots. Whether you are a pantser or a plotter, your characters have histories and quirks that lead them in certain directions, giving birth to subplots.
A good subplot tells us things about characters that we need to know in order to strengthen the main plot. It helps develop three dimensional characters, and encourages showing rather than telling.