The .Param method on .Page looks for the given KEY in page parameters, and returns the corresponding value. If it cannot find the KEY in page parameters, it looks for the KEY in site parameters. If it cannot find the KEY in either location, the .Param method returns nil.
Site and theme developers commonly set parameters at the site level, allowing content authors to override those parameters at the page level.
For example, to show a table of contents on every page, but allow authors to hide the table of contents as needed: