If we catch most of the fish of reproductive age, then after the current fingerlings grow up, there won't be another batch to replace them. We may find the fishery crashing a few years later, and then recuperating. A theoretical biologist formulates models, math, and computer programs to make predictions about how the fish population will change. The equations may have chaotic solutions, which means that what happens is sensitive to small changes. The theoretical biologist may be driven to politically unpopular conclusions.
Fisheries represent but one small part of our environment.
We need theoretical biologists working on the problems of our impact on the biosphere, and what we could do about it. With the environmental changes and plant and animal extinctions we are causing, time is short, and we need answers soon. Theory is essential, because we have only one earth to experiment with. Consider a medical problem. We know that x-rays can cause cancer, yet they also provide marvelous images of the inside of the body.