Why do they have to wear knee pads for that matter?
I watched so many matches and I always wondered why the bye runners ever have to wear a helmet?
As per the cricketing rules,
The player acting as a runner for a batsman shall be a member of the batting side and shall, if possible, have already batted in that innings. The runner shall wear external protective equipment equivalent to that worn by the batsman for whom he runs and shall carry a bat.
Helmets,pads and other things are external protective equipment which is worn by the batsman needs to be worn by the runner too.
Note: Currently bye-runners rule is not available. The rule is removed since it was used in a wrong way.
The role of a runner is a replacement for the injured batsman. In the interest of fairness to the fielding side, the runner must wear all the external protective equipment worn by the batsman he is running for, and must carry a bat.
This is obsolete now. Runners have been abolished from all forms of international cricket.