These are a better class of wood than the Pine Wood Plank and are commonly used in the creation of those better midrange buildings and upgrading of your basic buildings into more effecient ones.
Hardwood Planks are used in some upgrades. For example, to upgrade a Mason from level 1 to level 2 costs 120 Hardwood Planks plus 100 Marble. Hardwood is use in moving some buildings. For example, to move a level 3 Farm costs 75 Stones plus 100 Hardwood Planks.