Hello Lucas,
Are you asking what purpose do fleas serve by existing? This is more of philosophic question. It’s usually provoked by irritation. The same question is often asked about mosquitoes and diseases. It could also be asked, “What are humans place in the environment?” All of nature would flourish without us.
I don’t know the answer to these questions. It seems beyond our grasp as humans. All I can do is speculate.
In a broad sense, fleas, like all species, simply fulfilled an available ecological niche. It’s believed fleas were once free-living, flying insects. Then they started living within the dens of small mammal, feeding on the debris there. Eventually they start feeding directly from the animal. They lost their wings, as they were no longer necessary.
All organisms, including fleas, are part of the food chain. Whether they are consumed by animals, microorganisms or fungi, they help keep nutrients flowing through the system of life.
Blood-sucking parasites are vectors for pathogens. They may help spread disease to re-balance populations that are out of control. For example, black death was spread by fleas.
Through a philosophic lens, perhaps irritating pests exist to help us overcome petty external annoyances, teaching us to achieve equanimity in the midst of a world we can’t control. In addition, after dealing with fleas, it’s easier to appreciate life when they’re not around.
I’m sorry I don’t have a satisfying biological answer for you.
Adam