Why body fat beats the scale
Body weight alone cannot tell muscle from fat. Two people at the same weight can look and perform very differently depending on their body composition. Tracking body fat percentage shows whether changes on the scale are coming from fat, muscle or water, which is far more useful for fitness goals.
Tracking it over time
Measure under the same conditions each time, ideally in the morning before eating, using the same tape and the same spots. Day-to-day readings will wobble, so compare weekly or monthly averages and watch the trend rather than a single number.