If 衔 means 'hold in the mouth', why bother adding '在嘴里'? I was looking up 醒酒石. I found:


  醒酒石：所谓醒酒石，即喝醉酒后，衔在嘴里可以醒酒解渴的石头。


When I look up 衔  I find:


  
  用嘴含，用嘴叼：衔枚。燕子衔泥。
  


If 衔 means 'hold in the mouth', why bother adding '在嘴里'?

(Also, I'd be interested to know, geologically, what kind of stone is 醒酒石. Could be very useful some mornings!)
What is being held? Majority of Chinese VP especially less common ones are disyllabic or as in the present case monosyllabic verb combined with adverbial preposition phrase (介词短语),if object makes clear that mouth is involved there is no need to mention it, e.g. (bkrs examples) 燕子衔泥│他衔着一个大烟斗,衔环相报(wounded bird holds ring),