Honey in Pokemon Brilliant Diamond and Shining Pearl is an Item that, if used correctly, will help you find valuable and rare Pokemon. Honey can be bought endlessly from a man north of Floaroma town in the Floaroma Flower Field and will charge you 100 Pokedollars for each Honey.
Honey Tree Pokemon
If used out in the field while standing in Tall Grass, in a Cave, and while Surfing, you will attract a wild Pokemon to your location and effectively have the same use as the Sweet Scent ability.
A much more lucrative way of using Honey is to slather it on special trees littered around Sinnoh. These trees are yellow and stand out from the others. Slathering sweet Honey on a tree and leaving it alone for around 12 hours and then returning, there will be a chance that a Pokemon will be in the tree. If you interact with it, the Pokemon will jump out at you. You will know if there is a Pokemon in the tree when the entire tree periodically shakes. Changing the ingame clock will not make the Pokemon appear any faster.
Note: If you slather Honey on a tree and leave it for 24 hours, the Honey and the Pokemon will be gone, so be sure to check before then.
The rare Pokemon that can only be found by slathering Honey on trees are listed below.