The Guru Granth Sahib is not ordered by author, by date, by subject or by any narrative. It is ordered by rag, the melodic measure in which a hymn is to be sung, and that single decision shapes everything about how the volume can be read.
The consequence a first-time reader notices is that nothing develops. Compositions by the first Guru sit beside compositions by the fifth, addressing the same concerns in the same measure, and then the same concerns return sixty pages later in a different measure. There is no argument being built across the volume and no place where the teaching arrives. A reader looking for progression will conclude the book is disordered. A singer will find it perfectly ordered, because the singer needs to know what to sing, and the answer is on the page.
That is the point. The Granth was compiled for a community whose central act of worship is the singing of these compositions, and its organisation serves that act rather than serving study. The rag is not a label attached afterwards; it is part of the composition, recorded in the heading of nearly every hymn, and to sing a hymn in some other measure is to depart from what was set down.
The arrangement also carries an argument about authority. Filing the Gurus' work and the bhagats' work together under a musical heading, rather than in separate sections by author, puts them in the same frame. The organising principle is what the hymn is for, not who produced it, and that has consequences the volume never states and never needs to.