1. The most obvious perk is wood floors are typically easier to maintain and clean, and have a longer durability and life that most carpets designed for rental units.
2. Hardwood enhances the overall image of the rental unit which could help it rent and sell faster down the road. You might not command a lot more money for the hardwood, but you'll be competing with other quality units.
3. A well-kept hardwood floor tells renters that you're serious about taking care of the property. In turn, they will believe that you will continue to do so while they are tenants.