actually, for those who want to lose weight through exercise, setting ur goal in term of body fat percent is much much better.
this is because when u do exercise, u may build up some muscle mass. however the muscle is much heavier than fat. as a result, u may gain weight although u r losing fat. if u primary goal is to lose weight, this may be discouraging and demotivating.

on contrary, if u set the goal in BF reduction, u will be sure that u come nearer to ur goal...

Just imagine you're eating some chicken. The lean flesh of the leg is tough. The yellow fatty bits are mushy and soft and squishy.

The weightiness of muscle is in the density of muscle tissue.