In fermentation and in general in a bioprocess you use cells or part of cells (i.e. enzymes) to obtain the desired transformation. Biochemistry can thus give you the necessary knoweledge of how the transformation can work. In a way you change the "black box" to something more obvious
Metabolic processes, alter kinetics of enzyme reaction to get the product of choice; networking between pathways and regulating them; mechanism of reactions; acid/ basicity requirement of reactions and so on.
@ Yannis Karamanos and Ismaeel Muhamed : Although both the statement are diversified but quite relevant to their respective scopes. I am looking forward more discussions to satisfy my query.