One common task accomplished with transforms.conf is the creation of new indexed fields. Indexed fields are different from extracted fields in that they must be created at index time and can be searched for whether the value is in the raw text of the event or not. It is usually preferable to create extracted fields instead of indexed fields. See Chapter 3, Tables, Charts, and Fields under Indexed Fields Versus Extracted Fields section, for a deeper discussion about when indexed fields are beneficial.
Indexed fields are only applied to events that are indexed after the definition is created. There is no way to backfill a field without re-indexing.