Using their downloadable client, you can create an Armory-like page of your character on the site. Then you can create a Wishlist duplicate template of your equipped gear and swap in pieces that you want to acquire to see exactly how it affects your stats, resists, damage ratios, mp5, crit chance, defense, etc. Great for planning on how to get that last bit of resilience to hit the cap or balancing your +heal and +spell damage.
Another function I like is the personalized quest page. You now have an offline list of all the quests in your quest log. Plan your next step to complete the quest and read up on any strats you'll need to know beforehand. Also, the site tracks all your reputation gains so you'll know how close you are to the next rep level for every faction in the game.
If you're like me and enjoy extensive planning and researching offline, I recommend checking out the latest improvements to WoWDigger. With CTProfiles down for now, its a great alternative. If anyone knows of other sites that provide similar services, feel free to list them in the comments.