Cluster munitions are highly effective at destroying massed formations of men and equipment. This forces the attacker to heavily armor up, which doesn’t work well against modern cluster munitions, and/or disperse forces across a very large area so that few targets will be within the kill zone of a cluster munition to limit their effectiveness.

They limit the ability of an attacker to mass vast forces in an area for a brute-force assault. Soviet doctrine was notorious for this kind of attack, so cluster munitions were developed as a solution. Russia still uses this kind of assault, so cluster munitions still work against their forces. Other militaries (e.g. the US) use doctrines that render cluster munitions ineffective.

Just about every weapon invented was developed to address a specific scenario where it is highly effective at shaping the battlefield. Some have become obsolete because those scenarios don’t occur in modern warfare, but there is still a lot of diversity in the practice of warfare.