Why does the kidney of a cow have lobules, why the kidney of a human hasn't any?
It appears that the invagination of the renal kidney is vestigial from development and unlike human and other mammals, the bovine kidney does not form a smooth outer cortex.


  The kidneys of the bovine do not lose their foetal lobulation.


-Renal Anatomy - Anatomy &amp; Physiology en.wikivet.net

However this is not related to its function.


  Despite what it’s externally lobulated appearance may suggest, the cortex of the bovine kidney is continuous and the kidney is of multipyramidal type.


-Renal Anatomy - Anatomy &amp; Physiology en.wikivet.net

If you want further information on the subject, The bovine kidney as an experimental model in urology: external gross anatomy; Carvalho, et. al. Cells Tissues Organs. 2009;190(1):53-8. doi: 10.1159/000159370 made measurements of the bovine kidneys and compared then to human measurements. This article is unfortunately behind a pay wall, so you will need academic access to Medline to access.

One observation that they do make is that


  There are few publications about bovine kidney anatomy. The available information is general, contains little detail and is limited to the classical texts [Bossi et al., 1909; Nickel et al., 1979; Sisson, 1981; Dyce et al., 2004].