Why rooting a mobile?
Because it is better. A lot of software available for standard computers is not possible to install because you are just on the user level. Imagine you would be using Ubuntu or Windows and would'nt have the administrator level. With root you may install system relevant software, read system data out of your phone, read logs, install adaway, firewalls or other tools or even install a complete custom firmware.
Is it possible to update the stock firmware after a root?
A clear yes and no. What doesn't work is an OTA update. System updates are simply not possible via OTA, since we have a rooted, custom system. But there is a way to update the phone without losing data. What you need is a rooted Galaxy S8 Exynos SMG950F mobile, a PC and the original charging cable.