The common advice is that lifting weights "burns more calories" or "boosts metabolism," and while there's a grain of truth in both, that's not actually the strongest reason to lift while losing fat. The real reason is what happens if you don't.
What happens to muscle when you only do cardio
Weight loss, at its simplest, comes from a sustained calorie deficit — and a body in a calorie deficit will pull energy from wherever is easiest to spare. If your only signal to your body is "move a lot" (as most cardio provides) rather than "keep this muscle, you need it" (as resistance training provides), a meaningful share of the weight lost can come from muscle, not just fat. The scale can drop steadily on cardio-only weight loss and still leave you with a body that has lost strength along with size — softer rather than simply smaller.
Why the scale is a poor measure on its own
A scale measures total mass — fat, muscle, water, whatever you ate that morning — and can't tell you which one changed. Two people can lose the same five kilograms and end up looking, and feeling, completely different: one lighter and softer, the other visibly firmer, because one of them was also lifting. Photos, how clothes fit, and simple strength benchmarks (can you do more push-ups than last month?) are usually more honest progress markers than the number on a scale.
What "toned" actually means
"Toned" isn't a separate, smaller kind of muscle — it's ordinary muscle sitting under a lower layer of body fat. There's no exercise that "tones" a specific area without changing fat elsewhere; that idea doesn't hold up. What actually produces the "toned" look people describe is the combination of resistance training (to build or keep the muscle) and a calorie deficit (to reduce the fat covering it). Cardio can help with the second half. It does very little for the first.
A sensible first month
Two to three resistance sessions a week covering the whole body, moderate weights taken close to — but not necessarily to — failure, and a calorie deficit that's realistic enough to sustain for longer than a month. Cardio is a fine addition for heart health and extra calorie burn, but it's the addition, not the foundation. If the goal is to lose fat and still look and feel strong afterward, the weights are doing more of the actual work than the treadmill is.