When I fire while standing I like to get my support hand as far out as I can for control. It's not a make or break feature for me as I do have standard carbine length ARs as well but I feel I can control the rifle better the farther out I have my hand (while pulling it into my body). I have some 12'' rails that I think are just as effective for this as the 15'' but I like options
especially since this handguard is so lightweight.