Why I should I care about "knox" warranty can I still root my S5?
Knox warranty void has been discussed for a long time but I can't find consistent results on what it actually means.  

Some sources say, that it only covers data security, a warranty that makes sure your data is safe and private, so tripping the Knox warranty void flag doesn't void your hardware warranty.
(where I live there is a law that hardware warranty is independent from software warranty, so they have to repair your device even if the software has been modified, given that the software was not responsible for the damage ie. cracked screen, blown up battery etc.)

Other sources say, that it completely voids your warranty and if you break your phone, you are left with an expensive paperweight.

All of them say, that tripping the Knox warranty void flag is permanent because it's written into the bootloader, though, some devs found a way to not trigger that flag while rooting and found a way to force the system to overwrite the memory using a modified bootloader that is larger than the original one.
All of those methods have a higher risk of bricking your device than thousands of root attempts. Rooting is kind of secure given that you only modify a few files to give yourself full control over your phone.