Its a way to get an extra mod on armor. like bloodwood gives 2 hpr, and heartwood has 5 or 6 random mods, most desirable being hit chance. If you really want to make an elite piece of armor, you imbue it, then enhance it. Both will buff resists higher than you can get than just imbuing, heartwood has the chance of giving hci.
Now before the forged metal tool, and even since it, making that perfect imbued then heartwood enhanced piece of armor takes alot of crafting the base piece, and is a gamble weather or not youll get the hci.
Theres tons of threads about this, and its pretty easy to find the exact wood properties on stratics and other sites for more detailed info.