Some templates return a cleaned-up copy of themselves upon substitution. This is known as the substitution trick or subst trick and is especially useful for removing unsupported parameters (which may cause a page to land in a maintenance category) or updating a template to use the current names for parameters instead of their aliases.
Module:Unsubst offers a straightforward tool to carry out the subst trick.
A prominent example of a template that can perform the substitution trick is {{Infobox album}} and some of its associated templates like {{Singles}} and {{Extra album cover}}. Other examples include most maintenance templates like {{Citation needed}} and {{Unreferenced}}, which when substituted will generally add the |date= parameter with the month and the year.