A default implementation that field types use when being assigned a value, to parse that value into the correct format for data storage assigned value when assigning values to fields directly, which allows field types to offer a very developer-friendly interface for assignment.
Note that this default implementation simply returns the value as it was assigned, which is appropriate for most simple fields.
The Related Term field type, for example, overrides this method to allow terms to be set via arrays or CSV string of term slugs, term names, or term IDs. This is important, since the related term field type requires values to be stored as “taxonomy:term_id” tuples, which is not particularly developer friendly.