system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it necessary for refs to stand in the path of a javelin throw. Can’t they just stand on the sidelines and jog over after it lands?
gpt
The javelin doesn't always land point-first and stick in the ground. When it doesn't, it kind of slides along the ground, but the meet official needs to mark it where it first touched the ground. To complicate matters, if it lands flat they're supposed to measure from where the grip hits the ground. If it lands tail-first, they measure from the tail. All of that is difficult to do when you're far away. 

disclaimer: This hasn't always been the rule and may not be at some levels/in some regions.