Making up your own music or playing around with a well known music sheet is like a drug
The goal of every musician, professional or just amateur is to learn not only how to read and play notes but to be able to create something their own – improvise. This ability will give you more than just fun, it will make you a better musician along with other benefits:
Trains your ears. Being able to hear the music is the key element in being a musician. You can learn how to read and play, but the true musical art is about being able to hear the notes, know how they work together. Trying out different combinations trains your ears and lets you know if you play the sounds you want or not.
Improvising helps you to recognize the patterns. Usually musician warm up by playing the notes from top to the bottom and vise versa. By doing it couple of times it becomes a routine and you no longer focus. But if you improvise you have to see if the notes are played well together, you refine your chord, scale and arpeggio concepts.
You think ahead. When you play by notes your main task is to follow and play each note in order to make one single composition. Improvising however makes you think about the song as a whole and anticipate the next coming note, will it fit the scheme or not.
It boosts creativity. Improvisation lets you go beyond the music sheet limits. It gives you freedom to express and change the next line the way You wish, and not the composer, cause now you are the one who is creating the music.