Body weight is noisy day to day — water retention, sodium intake, and even sleep can swing it by a kilogram or more. Used alone, it tells you far less than people assume, and it tells you nothing about composition.
Performance metrics are often a better early signal: the weight on the bar going up, an extra rep at the same load, or recovering faster between sets. These move in a fairly direct line with real physical change, which the scale doesn't.
Progress photos and simple measurements (waist, hips, arms) catch composition changes that the scale can mask entirely — it's common to look visibly leaner while body weight barely moves.
This is why a coach asks for several data points instead of one. A single number can't carry the weight of telling you whether the plan is working.