How can making personas help in usability testing?

It gives a lot of context to the design process. Personas help you imagine who are you designing for. Personas are a representation of your key audience. So that you craft the experience to match these personas. It will guide you while wireframing and setting a style guide, you know building something for a white-collar employee is different from building something for high school graduates. It will also hel with information architecture and copywriting. 

Also, Marketers, business developers, and sales rely heavily on personas in their targeting and decision making.