User blog comment:Mrlmm0605/Let's Talk Bleach! - Part 13: Bankai/@comment-11446176-20160418015621/@comment-26091912-20160421080136

His hyper-speed enhances or complements his Shunpo, which allows him take make lots of afterimages. In addition to that, his attacks and reflexes are also faster; such as when he was able to slice through all of Byakuya's petals, avoid Ulquiorra's attack, and overwhelm Quilge so that he couldn't attack back. And sure, his strength is nothing crazy, but you gotta admit, stopping Yammy's punch and slicing his whole arm off with just one hand is pretty impressive.

If you simply meant his robes were just for aesthetic purposes, then the same can be said of all Bankai that change the wielder's appearance.