A server that answers needs somewhere to keep what it knows, and its own memory is the wrong place.
Database, briefly
A program whose whole job is keeping data safe on disk and finding it again quickly. Your server sends it questions over a connection. When your server restarts, its memory is gone, and the database still has everything.
Agents building a quick demo often keep data in a variable, because it runs first time and every check passes. Then a deploy wipes it, or a second copy of the server gives different answers. The "Data and failure" chapter goes much deeper.
“You stored signups in an array, so they vanish on restart. Move them to a users table in Postgres, read the connection string from DATABASE_URL, restart the server, and show me the row is still there.”