I don't know if this differs between universities and fields.
Bench fees, are a pot of money to fund extra costs associated with your research. So like lab consumables, software, conferences etc, that are part of your PhD but have costs. I am in engineering and get £5k a year for to cover all my lab work and conferences. So when I want something, I ask my supervisor for approval and the money comes out of the bench fees. You can think of it as a pot of money that you can spend on your PhD.