system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why is it beneficial to measure the depth of water in "fathoms", rather than feet or meters?
gpt
Sailors used to measure how deep the ocean was by dropping a weight at the end of a long rope. Then, when they pulled it in, they measured how far down it went by how many arm-spans worth of rope they retrieved. 

One arm span, or about six feet, was deemed to be a fathom.

The measurement isn't necessary anymore, but it lives on.