When baby powder is applied to your face the baby powder on your face helps to reduce acne and pimples by drying them out.
Baby powder when applied to your face or skin has the naturally ability to dry out the skin and reduce natural oils from excreting out of your body.
Just like when baby powder is applied to a babies diaper area during a diaper change the baby powder helps keep the babies skin dry by absorbing the moisture such as urine that comes into contact with the child's skin.
That was more of a problem with cloth diapers since cloth diapers kept the child's skin wet and not as dry as disposable diapers do now.
With disposable diapers the child does not feel wet unless the child has a leaky wet diaper and so baby powder on babies skin is really not needed.
But baby powder when applied lightly and gently to the face of an adults or teens skin can help reduce and prevent acne.