How do online stores get organic traffic to product listing pages?

There's this store in my area that has most of its traffic from organic sources. At the same time, they don't have many backlinks and their pages are not that optimized. Moreover, their most popular pages turn out to be product listing pages.

The last part is especially puzzling, as to my understanding a product listing page is not something you can really optimize for SEO - they have a variety of unrelated keywords and don't have a lot of text.

So, what could be their secret?