it's called the food chain and insects( aside from plants) are the biggest part of that. In other words if they went we all die or at the very lest suffer a great decrease in the number of living organisms.
Thank God I know my sins are removed by the blood of Jesus. All of them, past, present and future. I'm thankful for that because when I comb my dog everyday I catch those fleas and dunk them in soapy water and rejoice at their demise as they sink to the bottom of the bucket. As for ticks I remove them with a flea comb and then stick them in a Kleenex, fold it over them and then crush them with the rounded bottom of a spoon until I hear them pop. - I really don't have a clear answer for why they are here other then this present earth in not the perfect one as that New Earth and New Heavens that are to come. The bible does tell us that the Lord Jesus did say he saw Satan cast down unto the earth and just what came along with him could have been a Pandora box of all kinds of parasitic creatures. Thankfully we do have medications that can help to lessen the problems of these types of parasites. One thing to do is rejoice that they will be no more after this present earth is taken out of the way for that New Earth and the New Heavens.