This module is used by Template:POTD row and other POTD templates to allow for substitution of the Template:POTD texttitle created when substituting a Template:POTD/YYYY-MM-DD template with the |texttitle= parameter.
For example, compare the result from {{subst:POTD/YYYY-MM-DD|texttitle}} with {{subst:#invoke:POTD titletext|main|YYYY-MM-DD}}