Washing your beard may seem no different than washing your head in practice. Using shampoo that's designed for your head seems like the natural choice because it's what we have available to wash hair with.
The problem with this convienence is that most shampoos are specially formulated for washing away mass amounts of natural oils that are secreted by our scalp beneath the hair. Your face skin is much more sensitive than the skin on your head and produces far less of the natural oils that the scalp produces.
Using a typical head and scalp shampoo will overdry the skin on your face causing itchy, flaky skin and poor beard health. Instead, it's recommended to use a shampoo designed specifically for the beard
I used dandruff shampoo on my beard, and it was awful. It made my beard crunchy feeling. I should have known since it does the same thing to my head hair.