Plants and trees produce pollen so that they can reproduce. Some plants are wind pollinated, ie. they release their pollen into the wind in vast amounts in the hope that some of their pollen lands on the flower of another plant of the same species. This can be completely hit or miss and success is dependant on the strength and direction of the wind.
Plants evolved another method that was targeted and works with insects as part of their reproductive process. By offering a reward of energy rich nectar, plants enticed insects to visit their flowers and coated the visiting insects in pollen. The pollen coated insects then moved on to the next plant and dropped grains of pollen whilst collecting their next reward of nectar. This symbiosis relationship was so sucessful that both insects and the plants proliferated.
Various Solitary bees, Honeybees and Bumblebees collect pollen for their own means. Pollen is extremely protein rich and is the only protein that bees collect for their dietary requirements.
Today, Honeybees collect and store vast amounts in their hives so much so that it can be cropped by humans. Whether or not humans need pollen for their dietary requirements is debatable as we are able to obtain protein from so many other sources.