It matters because if the braces are not syntax, you can decide what code to execute as the condition body at runtime.

    set condition-body {puts "Hello, world"}
    if { $condition } $condition-body

To get this to run, you need to splice in the condition-body like so,

    if { $condition } {*}$condition-body

But you get the point.

But the "then" and "else" parts of the Rust "if" are expressions. What you're talking about doesn't seem to be related to which things are expressions, it's more like being able to eval code at runtime.