designed for a job, and excellent at it
Some yarns are generalists. Specialist yarns are not. These are designed to excel at a particular task: durable sock yarns, sturdy weaving yarns, hardworking cottons for dishcloths and utility knits. When used for their intended purpose, they perform beautifully.
Specialists are happiest doing the job they were designed for. Using them outside that role can lead to unexpected results: less elasticity, different wear, or a fabric that behaves differently than you expect. Swatching is especially helpful if you’re planning to experiment.