Customers, according to the article, can find there Old-fashioned sweets like $2.95 boxes of Gobstoppers, $1.95 chewable wax lips and three-for-35-cents 
Pixy Stix.
The reason is very simple - another way to generate income, especially from those who just browse the books in the store, as Fred Bass, Strand co-owner, explains on the article (he's the one standing on the photo above in front of the candy shelves).
Candies are of course just the tip of the iceberg when it comes to considering a visit in the store. Located in 828 Broadway (at 12th  St.), this New York's independent landmark bookstore is one of most famous bookstores in the world. As mediabistro.com: GalleyCat reminds us Strand "boasts an 18-mile collection's worth of new, used, rare, and  out-of-print books. Some of the rare titles include a first-edition of 
Lewis Baltz's 
Park City, a signed first-edition of 
Hunter S. Thompson's 
Kingdom of Fear, and a signed complete set of the 
Writings of Mark Twain: Definitive Edition. The last collector's item will set a buyer back $5,500."
And of course 
customers at Strand can plant a tree for every book they buy   there and receive our sticker at the counter!