Because one is a CNS stimulant and the other is a CNS depressant (they counteract each other). I used to take alprazolam 1mg whenever I took too much meth (~30mg) when I'm experimenting with doses and ROA, I will start to have tachycardia (and sometimes acute hypertension). Alprazolam resolves anxiety and tachycardia (although acute hypertension would still be there), but after about 4 hours, I started to feel anxiety again (since alprazolam leaves the system quicker than meth) so I dose another 1mg as long as meth exists my system. I don't think taking something that counteracts is good for cardiac health in long term (but I don't do it anymore), but benzodiazepines are the first line of treatment for stimulant overdose in ER.