The protein rest used to be mandatory in order to supply yeast with nutrients. This is no longer the case. Protein degradation during the malting process has all but eliminated the need for a protein rest in order to supply yeast nutrients. However, a protein rest in order to reduce or eliminate haze is still arguably necessary with most mash grists. Furthermore, the degradation of these large proteins will aid in sparging.
Large proteins are gummy and are bound up with starch.
By degrading these proteins you will loosen the mash bed, which will aid during sparging, and release more starch in preparation for saccharification. At the low end of the range the peptidase enzymes will be active to some extent, reducing medium proteins to simpler constituents. Additionally, the protease enzymes will be active for a longer period of time resulting in more thorough degradation of large proteins. A beer with a light body and a delicate character would benefit from such a rest. The protease enzymes will degrade the large proteins efficiently enough to reduce or eliminate haze.