Sufficient protein in the diet is needed to protect lean muscle mass. It is also needed if you intended to stress your muscles through exercise in order to build muscle. This calculator uses body fat percentage to calculate lean body mass. This is the way the keto macro calculator knows how much protein to suggest.
Body fat is most accurately measured through a DEXA scan or hydrostatic weighing. This can be useful information to have before starting a new keto diet and lifestyle so you can more accurately track changes over time.
However, for the purposes of this calculator it is fine to estimate body fat as it does not significantly affect the calculations. If you are estimating body fat, err to the high side as higher body fat gives a goal of less calories to lose weight. Underestimating body fat will give a greater calorie goal.