Variables2[expr] is like Variables, but it also works on rules and equalities as well as lists thereof.
Variables2 always applies Union to the output.
Some cases where Variables2 is much more useful than Variables
Variables[{a -> x1 + y1, b -> x2 + y2}]\{\}
Variables2[{a -> x1 + y1, b -> x2 + y2}]\{a,b,\text{x1},\text{x2},\text{y1},\text{y2}\}
Variables[a + b == c + d]\{\}
Variables2[a + b == c + d]\{a,b,c,d\}