If you took piano lessons, you learned that the piano is the whole orchestra. Your teacher made you play legato until it was sweet as strings. You hammered counterpoints out as bright as trumpets. You laid down the bass while adding percussive elements as well. You may have even learned how to voice individual instruments within a hand, so that the melody would sing out above the accompaniment.
So what happens when you start playing with other instruments? Hopefully you are adjusting your groove. Hopefully you’re not playing the same way you did before. Because if you are, there isn’t any room left! You are the whole orchestra when playing solo, however, when playing with a bassist, you really shouldn’t double his parts. He is going to be a lot better at laying down that low end and voicing it with respect to what you are playing than you will ever be. And that bassist is going to be able to groove against what you are playing in time, creating a more compelling momentum. When playing with an acoustic guitarist, you really shouldn’t be doubling the rhythm. You’re never going to get a feel as good as he will get, partially because you don’t really have upstrokes/downstrokes the way he has strumming. Likewise, when playing with a string section, or playing with a percussion section, or playing with a guitar, or playing with a choir… each new element that is introduced means you need to play less – or it just becomes a big mess.
Not doing this is how you get the “Wall of Sound” that is the bane of every sound tech’s existence. “You must unlearn what you have learned.”