Why do we surround the argument of the Laplace transform operator with $\{$ and $\}$?

Question

Is there a special reason why we write $\mathcal{L} \{ x\}$ instead of $\mathcal L (x)$? Do the curly brackets have some sort of special meaning (mathematical or notational/conventional)? I think the only other places I've seen curly brackets intentionally used is when denoting multisets, which I'm pretty sure is not relevant here