Why not just let statements return value?

Statements in many Languages can return value directly, for example, Rust:

```rust
let flag = if (cond) {
  "True"
} else {
  "False"
}
```

So why we need the keyword `do`?

Even `{...}` is better than `do {...}`

```
let flag = {
  if (cond)
    "True!"
  else
    "False"
}
```

These syntaxes will not cause ambiguity. Am I right?