These are items that cause certain Pokemon to breed differently. For example, the Pure Incense can be used to get you a Chingling, otherwise, expect to get Chimecho eggs instead. They also have certain effects when held by a Pokemon, as well, such as boosting certain types of moves, or making wild Pokemon less likely to appear. While in Pokemon Go, they make it so Pokemon are more likely to appear.
Did you use the Incense items in the Pokemon games?
I remember wanting a Munchlax in Pokemon Diamond, but didn't have a Full Incense, so boy was I confused as all hell when I had a Snorlax hatch from an egg! But, I've done it before, yes. I always breed Roserade with a Rose Incense in mind, because I don't want to start off too easy w/a level 1 Roselia, you know?