What is an important ecological role of nematodes that are beneficial?

Nematodes that are beneficial indeed play a significant role in controlling pest populations, which is why this answer is appropriate. Beneficial nematodes are often used in biological pest control strategies as they can parasitize and kill various harmful insects such as larvae of beetles and other grubs. By doing so, they help maintain the balance within the ecosystem by managing pest populations naturally, which can reduce the need for chemical pesticides and promote healthier soil and plant life.

While options regarding attacking plant pathogens, decomposing organic material, and providing nutrients to plants touch on other important ecological processes, they do not specifically highlight the pivotal role nematodes play in pest control. The primary application of beneficial nematodes in agricultural and gardening practices revolves around their effectiveness in reducing pest populations, making this answer the most relevant within the context of the question.