The beneficial effects of long-term calorie control

"Best practices" for healthy living are constantly being updated as scientific evidence bolsters or overturns existing knowledge.
It used to be that if someone wanted to lose weight - which in most cases referred mostly to losing fat - typical advice would be to eat as little as possible and do a lot of cardio-based exercise such as jogging or swimming.
While this approach generally worked for most people short-term, many found themselves in the "weight loss yo-yo", where they might indeed lose weight, but gain it back (and then some!) very quickly after returning to a more "normal" diet.
As such, weight loss advice began to focus more on metabolism and resistance training. As muscle is associated with metabolism, building muscle mass increases metabolism, and thus the amount of calories the body expends at all times. Therefore, it's harder to "overeat" (consuming more calories than you expend) and so it's easier to keep weight off if you have more muscle. So the focus began to shift away from reducing caloric intake, and more upon building up muscle first before worrying about reducing calories (sometimes referred to as "bulking" and then "cutting").
At the same time, don't entirely discount the value of cutting calories! Long-term, it does appear that reducing caloric intake slightly over a long-term period still has benefits in of itself. The CALERIE (Comprehensive Assessment of Long-Term Effects of Reducing Intake of Energy) study looked to study what would happen by testing the effects of caloric restriction through a clinical trial.
The goal was for test subjects in the Caloric Restriction group (CR) to complete two years of 25% calorie restriction (based on their "baseline" energy requirements at the beginning of the project), designed to achieve about 15% weight loss during the first year. There was also a Control Group that simply continued their current diets without any restrictions. Neither group was mandated to undertake any form of physical exercise.
Over the two years of the study, although the CR group was only able to average a 14.8% reduction in caloric intake (and not the original 25% goal), they were still able to achieve noticeable weight loss results after year 1, which was maintained by year 2. In contrast, the Control Group generally gained weight:

Study/chart source: Most, J., Gilmore, L. A., Smith, S. R., Han, H., Ravussin, E., & Redman, L. M. (2018). Significant improvement in cardiometabolic health in healthy nonobese individuals during caloric restriction-induced weight loss and weight loss maintenance. American journal of physiology. Endocrinology and metabolism, 314(4), E396–E405.
In addition to weight loss, it was found that Caloric Restriction for two years resulted in beneficial effects on body composition fat distribution, and cardiometabolic risk factors. Now, the study certainly wasn't unique in demonstrating the benefits of diet-induced weight loss in reducing health risks. However, it was good news to observe that significant improvements could be made in body composition even through relatively small (14.8%, or about 300 fewer calories a day) reductions in caloric intake, and not necessarily harsh and unsustainably large cuts. That's basically the equivalent of skipping one sugary drink, or a small bag of chips each day!

Using a validated body composition analyzer makes it easy to regularly track progress and make adjustments as needed, in a more precise way than a bathroom scale can, because a bathroom scale doesn't distinguish between fat and muscle and water.