In this case, I have no idea why they did it. Maybe to ease the transition from C-like languages to an ML language. I personally have never seen anyone use braces in F#.

Optional syntax is not at all rare, though.

Examples from various popular languages: white spaces, braces around single statements, trailing commas in lists, inferred type declarations, and parentheses.

There are also multiple ways to write things, like if...else vs. ternary operator vs. pattern matching vs. switch, often in the same language (C# has all of those, with pattern matching only being added recently due to demand).

I agree that I'd prefer that there only be one way to do things, but that's easier said than done. Sometimes users demand new syntax for different contexts or for the sake of terseness.