Why do URLs use slashes?
The short answer
Slashes separate hierarchical parts of the path after a domain and make a web location easier to describe.
How it works
In /next-why/ja/, the first slash marks a path from the site root and later slashes separate segments. A URL path does not have to match a real folder.
A familiar example
Some sites treat a URL with and without a trailing slash as different, so consistent links matter.