It's for the taste of peanut butter with less calories. I also personally prefer it in shakes, because I feel that it blends better than regular peanut butter.
There is nothing wrong with carbs before bed. There is no greater fat storing risk. It takes a surplus of calories to gain weight, not carbs before bed.
It's all about progressive overload to build muscle. You can either overload your muscles by adding reps or increasing weight and increasing intensity. difference things work for different people.
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