The concept of font-family is that different devices support different fonts – and the idea is to provide a list of fonts that the system attempts to use in the specified order.
Typically, the list starts with your preferred custom font and ends with one of the super common fonts as the final fallback.
This is important to support different kinds of devices. Like if someone is using an obscure Android model, it may not support your favorite font – and this lets us specify the "next best choice" preference.
As a best practice, you want to specify your favorite custom font as the first choice in this "fallback order". Then, look up whether your ideal font is a Serif or Sans-Serif font – and use either ..., serif; or ..., sans-serif; as your final fallback.
