Adding text or "supers" to your video project gives it another element and adds depth. Text sometimes can send a message much more succinctly and clearly than a narration. It can reinforce narrated information or remind viewers about the people in your piece and the message you're trying to convey.
The text tools in Premiere are similar to those in standard graphics/text programs, with several extra features tossed in. The infinite customizability of your text's appearance means you can create a "look" unique to your productions.