Why bears rub against trees

We had always understood that it was to scratch itches or to mark territory, but a New York Times article offers another possibility:
When bears wriggle against bark, the tree scratching posts leak out resins and saps. These substances stick to fur and skin , and are water-resistant, making them a strong contender for an effective tick repellent...
For years, biologists have observed that brown and black bears have an affinity for certain types of trees — especially conifers. They are also attracted to beech tar, which is a useful experimental proxy for substances leaked by trees. The appeal of the tar is so strong that scientists use its sticky, strong scents to attract bears for studies or to call them inside in zoos...
“Repelling parasites is probably not the primary function of tree rubbing, but it certainly could be an additional benefit.”