From the book:

Dynamic proxies make code easier to maintain. Instead of a large number of handcrafted classes, we write a single dynamic proxy. It is the “don’t repeat yourself” principle on steroids.

For example, one company managed to replace 600,000 generated code statements with a single dynamic proxy. They had written a magic tool to migrate their proprietary language to Java. Their business system had hundreds of entities. Their tool converted each entity to several Java classes. However, after this initial generation, programmers had to maintain the code by hand. Maintenance was laborious and error-prone. A single dynamic proxy replaced all that. Less code means easier maintenance.