Why Square Brackets for Expectation

Question

I've often seen $\mathbb{E}[X]$ instead of $\mathbb{E}(X)$, but it seems variance is almost always $Var(X)$. E.g., Wikipedia for Expected Value and Variance. Is there a good mathematical reason for using square brackets for one and not the other?

Answer 1

The random variable $X$ is a function whereas the Expectation is a functional. Mathematicians adopt the use of square brackets for functionals. This is why.