[Saurreg]

I can think of several reasons why Eru inflicted suffering among the population of Middle-Earth.

1. He was a sadist. He derived self gratification by inflicting pain and making the sufferers acknowledge his supremacy without question. A case of absolutie devotion and obedience, or else...

2. Evolution. Survival of the fittest. Eru did what he had to to weed out the weak and ensure that only the most adaptable inherited Middle-earth. A good investment for the future.

3. Preparation for the great beyond. This applies to men. Eru never stated that his final gift for them beyond Arda was going to be a bed of roses. The trials and tribulations could have been a form of training or selection process to prepare men for the afterlife.