It's not so much that this ability is specifically put in for some reason. It's just something that falls out of several other things.
Rust chose a "curly braces and semicolons" syntax because that's the sort of syntax that is normal in the sorts of PL spaces Rust wants to be used in. I am not sure exactly why being expression-oriented was chosen, but if I had to guess, it would be due to that just generally being considered a better option among many people at the time it was chosen.
So okay, you want expressions, and you want semicolons. Therefore, "you separate expressions with semicolons" is a pretty natural outcome. And since we're expression oriented, "a block is an expression that evaluates to the final value" is near tautological. And since it's an expression, it can go anywhere an expression can go.
Not being able to do this would mean creating specific restrictions against it, and then having to memorize when things don't follow the usual rules. That's more complicated than just letting expressions be expressions.
It's not so much that this ability is specifically put in for some reason. It's just something that falls out of several other things.
Rust chose a "curly braces and semicolons" syntax because that's the sort of syntax that is normal in the sorts of PL spaces Rust wants to be used in. I am not sure exactly why being expression-oriented was chosen, but if I had to guess, it would be due to that just generally being considered a better option among many people at the time it was chosen.
So okay, you want expressions, and you want semicolons. Therefore, "you separate expressions with semicolons" is a pretty natural outcome. And since we're expression oriented, "a block is an expression that evaluates to the final value" is near tautological. And since it's an expression, it can go anywhere an expression can go.
Not being able to do this would mean creating specific restrictions against it, and then having to memorize when things don't follow the usual rules. That's more complicated than just letting expressions be expressions.
(also your let is missing a semicolon)