What problem does log rotation solve, and how do you configure it?

Without rotation, the json-file driver appends to a single ever-growing log file per container, which can exhaust host disk space over time for long-running, chatty services. You configure max-size and max-file either globally in /etc/docker/daemon.json or per-container via --log-opt, capping both individual file size and the number of rotated files retained.