Terbium's sole claim to fame is that combined with iron it makes an alloy that shrinks and expands depending on the strength of a magnetic field. This property has been exploited in acoustics research. But other than that terbium really is an otherwise obscure member of the rare earth group - and pretty expensive to boot. 

Chinese refining facilities are by far the biggest rare earth producers with the U.S., Australia and Russia far behind despite having ample deposits. In any case, terbium sales make up only about 5% of the rare earth elements trade indicating both its relative rarity and high cost.