Work, in physics, measures how much a force actually accomplishes by acting through some distance, and energy measures a system's capacity to do that work. The work-energy theorem connects the two directly: the total work done on an object equals exactly its change in kinetic energy, its energy of motion. That single relationship lets a physicist find an object's final speed directly from the total work done on it, without ever having to track exactly how the force varied moment by moment across the entire motion.
Applying Newton's laws directly can require tracking a force's every changing detail
Solving a motion problem directly from Newton's laws typically means working out an object's acceleration at every instant and then integrating that acceleration over time to find its changing velocity, a genuinely demanding calculation if the force acting on the object changes in complicated ways as the object moves, growing stronger or weaker depending on position, say. That approach requires knowing the force's exact behaviour throughout the entire motion, not just its net overall effect.
The work-energy theorem replaces that whole calculation with a single total
The work-energy theorem sidesteps that demanding calculation entirely: since total work done equals the total change in kinetic energy regardless of exactly how the force varied along the way, a physicist only needs the total work delivered across the whole motion to find the object's final speed, without ever calculating the force's instant-by-instant behaviour at all. This is exactly why the work-energy theorem is such a genuinely powerful shortcut for problems involving a varying force: it converts a potentially difficult, detailed tracking problem into a single, much simpler total-energy calculation.
What we're still unsure about
That total work done on an object equals its exact change in kinetic energy is well established, rigorously derived directly from Newton's laws and confirmed across the entire history of classical mechanics. What's more genuinely a matter of practical technique is exactly how to actually calculate total work when a force varies in a genuinely complicated way with position, since that calculation itself can require real mathematical care, integrating the force over the exact path travelled, and physics students and engineers alike continue to develop and rely on a range of calculational techniques and approximations for exactly these more complicated, varying-force cases, rather than the work-energy theorem alone making every such problem trivially easy.
This sits inside Work, Energy & the Work-Energy Theorem, one of eight topics in Mechanics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.