Just by rooting your phone you add to your Odin download counter, which some bored tech who is truly digging will notice right away. This is why triangle away is recommended before odin-installing the stock ROM. Unless it's been updated, triangle-away doesn't work with all S4 models yet.
And the reason for the full ROM download is because root permanently alters some files that even unrooting won't change back. Additionally, unrooting won't set the Device Status back to "Official" instead of "Custom" (Settings -> More... -> About device -> Status -> Device Status)
Samsung Galaxy S® 4 MJA Rooted with CF Auto-Root Tutorial
HTC EVO Design 4G™ v4.0.3 Bootloader Unlocked & Rooted with HTC SuperTool v3
If it ain't broke, use a sledge hammer.