Preferences are materials, creatures, or objects described as being liked by a dwarf in their profile. A dwarf's preferences (and disliked vermin) tend to feature in artwork the dwarf makes more often than subjects not listed in a dwarf's profile. For example, a mason that likes cows for their haunting moos and absolutely detests bats will make statues of both. This is presumably out of love for cows and horrified fascination for bats.
 Benefits to Productivity
A dwarf working with a material they like will produce higher quality goods. For example, a mechanic that likes olivine will generally make better olivine mechanisms than granite ones. This is most important during embark, when individual personality profiles and skills can be best matched up according to your specifications.
Want high quality bedrooms? Make the dwarf that likes beds your carpenter. (For some reason, only a few dwarves love beds, others just live with mutual acceptance of them.) Want to build wealth absurdly fast? Make the dwarf that likes gold your blacksmith. Want to outfit your military with classy armor? Urist McLikesbreastplates would be a good candidate for the job.
 Value Considerations
A mason who likes horses will often make statues of one or two horses, or even statues of themselves admiring horses. This can be great if the mason is chronically depressed: surround their bed with images they like, and they will be happier.
This phenomenon is particularly important when choosing an engraver: although rare, some dwarves do not detest vermin, and will therefore do things like fill your rat-phobic mayor's office with masterfully designed images of rats.