What are God’s reason why He created different insects and even rats and
cockroaches?

I would not presume to speak for God on why he created rats
and cockroaches. I would say that from a biological perspective every
creature has its part–even bacteria and other microbes which can at times
cause diseases–are essential elements in creating a finely-tuned
ecological system. Bacteria serve a number of absolutely essential
functions; breaking down cellulose, turning nitrogen in the air into
useful nitrogen for proteins and so forth. We cannot live without
bacteria! Every creature has its place and “use” within the natural
God-created ecosystem.

Rats and cockroaches, as well as spiders and mosquitoes are as
necessary to a functioning ecosystem as tigers, fish, trees, flowers and
so forth. It is true that in an unnatural environment such as a city or a
human habitation, rats and cockroaches become pests, but in a well-oiled
ecosystem they fit into the beautiful harmony and cycles of nature. Most
of the living things we perceive of as pests were made into pests by human
intervention into and destruction of the natural system created by God.

In Psalms 19 David talks about how nature declares the glory
of God. I believe that even those creatures which we despise, such as
snakes, rats, worms and so forth are awesome and beautiful creations of
the almighty God.

John Oakes, PhD