The log is where the actions of WPI are documented.
Every notable action that WPI does, it logs to this text file. When you visit theLog page inside the plugin’s admin area, it’s this text file you’re seeing.
Random web visitors can’t see this text file on your server because it’s blocked with htaccess for security. It only loads inside the WPI plugin.
The log file is periodically trimmed if it grows larger than 10k lines. This is simply to stop it growing to hundreds of MB in size, requiring “log file rotation”. So only the last 10k entries are kept (which is more than you’ll ever need to look at).