The Major scale is certainly the most important scale in music (at least for Western music), not only it is used in many styles but a large part of music theory and harmony are built on how this scale is constructed.
To improvise on a chord progression, we first have to analyze its harmony based on the different chords it's composed of, which will allow to deduce the scales to be used at the level of the Improvisation and solos. For this, it is necessary to master the system of harmonized scales, whether for major keys or for minor keys, in order to detect whether all the chords of the grid are diatonic to the same key, or if this grid includes is composed of chords (modulations) foreign to the main key. Let's us take for example the C major harmonic scale, which includes the following chords: C Dm Em F G Am Bdim Si tous les accords d'une grille sont diatoniques à cette tonalité (c'est à dire qu‘ils appartiennent à la gamme harmonisée de Do Majeur), il sera donc possible d’improviser sur tous les accords avec une seule et unique gamme, dans le cas présent la gamme Majeure de Do. If all the chords of a grid are diatonic to this key (ie they belong to the C Major harmonized scale ), it will be possible to improvise on all the chords with a single and unique scale, In this case the C Major scale.