Gaining access to the internal file system.You can use the access to install custom kernel or ROM.Some phone show some sign at boot screen showing that it's running custom rom.That's how some shopkeeper find it.
Honestly majority of the ori handset repair center don't even care whether you're rooted or not.If you root & install custom rom sometimes they just close 1 eye & repair it FOC.
IMO rooting & using custom kernel/os should not void warranty.Pushing the hardware like excessive OC or something like that is reasonable cause to void warranty.
This is the first seller that know who really against the rooting.