Okay I see why now. An XNOR gate outputs true if and only if both inputs are the same. So an XNOR gate can test the equality of the inputs.
It seems there are some overlapping fields that use boolean algebra (math, digital circuit design, logic, computer programming; obviously all extremely related.) Each interprets the boolean concepts slightly differently depending on their respective problems. Like xnor seems almost exclusively used in digital circuit design. Maybe because it’s more of a literal description of what the circuit is: a negated xor gate. The equality operation is like one level of abstraction above that, and more useful for most purposes. This is mainly me getting down some thoughts.