How did the Variations on a Theme by Haydn help prepare Johannes Brahms to write his first symphony?

By the 1870s, Johannes Brahms was moving with great deliberation toward purely orchestral composition. His 1873 work, Variations on a Theme by Haydn, served as a crucial experimental step in this process.1

Even the famously self-critical composer considered the orchestral version of this work to be a completely successful endeavor. By mastering this complex form, Brahms proved to himself that he possessed a full command of the symphonic idiom. This success gave him the confidence to finally embark on the completion of his Symphony No. 1 in C Minor, which he finished in 1876.1