I see it this way, the full signature for defining a variable is:

    var foo: Foo = Foo{};

There's two ways to shorten it:

    var foo = Foo{};
    var foo: Foo = .{};

It can infer the type of the var from the right hand side; or the type of the right side from the type of the var.

So when you see .{} as an argument, it is inferring the type from the function signature. It happens to be empty only because it's using default values (or is a tuple with 0 items).

Edit: fixed the extra dots.