Question: As a pianist who have only experienced classical music yet, I recently got my hands on [The jazz theory book](https://rads.stackoverflow.com/amzn/click/1883217040), because I am curious about this kind of music that is totally new to me. 

The theory is focused on chords progressions and the scales that complements them, which led me to wonder a few things : 

* Why choose to describe music with chord progressions ?

I mean, it doesn't carry that much informations. The chords themselves can be played in a million possible ways (with inversions, arpeggios, rythm, voicing, dynamics, etc.). 

I understand that different chords convey differents feelings or emotions, and can be descriptive of the particular progression of a piece but there is so much more to it !

* How exactly do jazz pianists use chords progressions when playing a lead sheet ?

I would be surprised to ear that they just play chords with the left hand while playing the melody in the right hand, as indicated in the sheet music.

Is the lack of informations given in a lead sheet the reason why improvisation is such a big part of playing jazz ?

In other words, do jazz pianists have to improvise over every bar of their lead sheets, by deciding on the fly of the way they are going to play all the (important ?) stuff that is missing on the sheet ?

Answer: 
> 
> Why choose to describe music whith chord progressions ?
> 
> 
> 

Keep in mind that a lot of Jazz is based on the premise of putting your own spin on existing songs (standards). The melody and chord progression is typically the only part that stays the same between those interpretations. And even those vary. 

So lead sheets convey only the parts you need to know—the kernel of the tune. Sometimes that *does* include more than just chords and melody like an intro or bass ostinato that is an important part of the tune. The rest is open to interpretation.

> 
> Is the lack of informations given in a lead sheet the reason why
>  improvisation is such a big part of playing jazz ?
> 
> 
> 

It works the other way around.

> 
> In other words, do jazz pianists have to improvise over every bar of their lead sheets, by deciding on the fly of the way they are going to play
> 
> 
> 

Yes, mostly. The way you *comp* a piece may not be true improv each time. *Maybe* you know generally what voicings you might play because you've played the tune before. But the rhythm is much more up in the air especially given that you should be interacting with the rest of the band and playing off of their rhythms.

But it doesn't *have to be* that way when you're starting out. Feel free to write out exact arrangements if that helps you.
" Why choose to describe music whith chord progressions ?

I mean, it doesn't carry that much informations. The chords themselves can be play in a million possible ways (with inversions, arpeggios, rythm, voicing, dynamics, etc.). "

And that, in a nutshell, is what jazz is about. Finding the million different things that can be played over a certain chord sequence. 'Jazz' can also include sticking to the composed melody and re-harmonising. Or in going off at tangents only vaguely suggested by the original song. But mostly it's based on the chord sequence. Great jazz musicians invent alternative melodies. Lesser ones just run up and down 'the scale of the chords'.