Have you ever heard a song that didn’t sound quite right? You couldn’t’ quite put your finger on it, but you knew something was wrong.
The melody was off. Or maybe it was the harmony.
Or maybe it was the rhythm.
When comparing the three, rhythm is by far the one thing that can change the way you play than all the others.
Why? Sit down and play music with no thought process about rhythm. Just pluck out the notes in a random order.
Not much of a song, is it? 
Rhythm controls tempo. It sets the speed. It also creates structure.
Rhythm is the technical part of the song. Have you ever mentally walked through a song, practicing it in your mind before you sit down to play? The best piano players in the world do this continuously in their minds. They see themselves on stage. They hear their music in their minds. And what improves it all is to keep a regular beat.
A metronome can help you keep that beat.
Try an experiment. Set the metronome for your desired song. Before you sit down to play it, play it in your mind first, following the beat. See yourself playing it. Hear it in your mind. Hear the rhythm. Feel how you play it.
Now sit down at the keyboard and play it for real. Do you notice it’s easier?
Also, try using a metronome when you’re first learning a song. Listen to the “tick tick” and adjust your playing to keep up with the beat. Play through difficult passages, playing again and again until you get it right. You can even use the visualization technique with the metronome keeping the beat.
Do you notice a difference in your playing?
