"Sightreading" means "to play at first sight." It's important that pianists be able to play music well the first time they encounter it. This is a skill that must be developed early on in a pianists journey and maintained throughout.
I provide my students with sight-reading books that contain songs designed to only be played once. The student plays the song, then moves on. That simple. There is no studying, no learning, no rehearsing. Just a quick play-through and on to the next piece of music. Playing through 4 or 5 short songs each day usually keeps the sight-readers eyes sharp.
Of course there are special technique "secrets" involved that allow a pianist to excel. I normally recommend early sight-readers begin by reading the letter names out loud. This improves note recognition and is a key element in being able to sight read. I also recommend they scan the song for dynamics and name them out loud, so that they are easier to remember while playing for the first time. Fingering is crucial so I have the students name the fingering out loud and note any changes to it throughout the piece. These three "secrets" give the student building blocks for success.
Pattern recognition is arguably more important than knowing specific notes. If a student can pick up a rhythmic or melodic pattern, notice when it repeats, when it disappears and arrives back in the song later on, this makes the process much smoother.