why do allergies exist?

They exist because of the way our immune system works. Essentially it senses things entering our body and triggering an immune response. This is useful in cases of bacteria, but less so in case of pollen. Generally our body has not way to no beforehand what is going to be harmful or not, but in most cases harmless things only trigger mild inflammation (or no response at all). However, if you immune system is overly sensitive (e.g. due to lack of exposure to things in hyperhygienic environments as postulated by the hygiene theory) an allergy is the consequence.

 

In extreme situations it may even mistake your own body as foreign and mount a response, and the result could be an autoimmune disease.

Very interesting, thank you