He wrote a letter to a music critic in 1939 and explained his reasons for arranging Brahms' work for orchestra:
"My reasons: I like the piece. It is seldom played. It is always very badly played, because, the better the pianist, the louder he plays and you hear nothing from the strings. I wanted once to hear everything, and this I achieved. My intentions: To remain strictly in the style of Brahms and not to go farther than he himself would have gone if he lived today. To watch carefully all the laws to which Brahms obeyed and not to violate them, which are only known to musicians educated in his environment."Perhaps another reason he did it was that at this time Schoenberg had already developed his "method of composing with twelve tones which are related only with one another". His method shook the world of serious music so he may have been trying to add legitimacy as a composer by orchestrating Brahms' work. He also gave a lecture and wrote a subsequent essay called Brahms The Progressive. Schoenberg's objective was “to prove that Brahms, the classicist, the academician, was a great innovator in the realm of musical language, that, in fact, he was a great progressive.” He considered Brahms his musical ancestor, along with Wagner, Beethoven and Mozart. His new method of composing was not so much revolutionary as evolutionary, at least to Schoenberg.
Schoenberg began his composing life as a Late Romantic, but his compositions showed signs of breaking with tonality early on. Even after he developed and used his method, he would lapse back into his earlier style, especially in his older years. Schoenberg was in some ways a paradox, as he expanded upon what Wagner and Liszt had begun while at the same time he championed Brahms as a progressive composer. In his own compositions Schoenberg could be conservative in form as in many instances he stuck with traditional forms used by Romantic composers.
Schoenberg was faithful to Brahms' original work in that he changed no notes. His brilliant orchestration is another matter. Brahms' orchestration was like his solo piano music; not outwardly brilliant and colorful, but complex and well written. Brahms' orchestration suited the character of his symphonic works perfectly. Schoenberg's orchestration of the work is radically different from what Brahms would have done. My post of Brahms original version of this work can be found here.
I wouldn't say that Schoenberg's arrangement suits Brahms' music like a glove, but it does have its moments. Shoenberg begins with a rather straightforward arrangement of the first movement, but after that each movement gets stamped with Schoenberg's style more and more. It leads to the final movement, an absolutely wild rendition of Brahms' Rondo alla Zingarese. With more and more percussion and quirky orchestral techniques, Shoenberg pulls out all the stops and makes music that is pretty wild in its original form completely over the top. Schoenberg had a liking for and understanding of Brahms' music, that much shows in the arrangement. Schoenberg's orchestration doesn't cancel out the greatness of the original, but it is interesting. And frankly, Schoenberg's last movement is incredible.