From a page move: This is a redirect from a page that has been moved (renamed). This page was kept as a redirect to avoid breaking links, both internal and external, that may have been made to the old page name.
To its plural form: This is a redirect from a singular noun to its plural form.
Redirects of this sort exist for reader convenience in cases of singular–plural pairs. It is also used for "false singulars", wherein the plural or plural-looking form is better attested in usage, such that the normal "prefer the singular" Wikipedia naming convention is not followed. Examples:
Use this rcat to tag qualified mainspace redirects only; if qualified singular forms are found in other namespaces, use {{R from modification}}instead.