The explanation given is that VS never intends to sell these bras, but as the jewelled bras continue to attract media interest (**around 33,300 google results), it brings the brand to attention of potential customers. Even if no one buys the bra, these jewels can be easily recycled.
Also, the presence of the bejewelled bras in the catalogue shifts the frame of reference that defines appropriate spending. In relative comparison, spending a few hundred dollars on another VS range seem so thrifty compared to spending US$5million!
*Apart from the Victoria Secrets problem, there are other questions such as “Why do women endure the discomfort of high heels?”, “Why do lawyers spend more on cars & clothing than university professors with the same income?”, “Why do humanities lecturers, who should be more adept than most in their use of language, often write so unclearly?”