> What do we gain by the shorthand?

Because it's new syntax, it allows for parsing the literal ahead of time and eagerly evaluating the substitutions. Code like

    bar = 42
    spam = t"foo {bar*bar} baz"

essentially gets translated into

    bar = 42
    spam = Template("foo ", Interpolation(bar*bar), " baz")

That is: subsequent changes to `bar` won't affect the result of evaluating the template, but that evaluation can still apply custom rules.