You must note that, in many cases, your examples above could produce different results.
URL helpers always refer to the Laravel application base path, and that's the main advantage IMO.
Let's say your application base is /larapp. Let's say you are returning a view for /some/route. Let's say the stylesheet is in /larapp/assets/css. You don't easily get to that without URL helpers:

<link rel="stylesheet" href="assets/css/app.css">
<!-- http://your.host/larapp/some/route/assets/css/app.css
NOT FOUND -->

<link rel="stylesheet" href="/assets/css/app.css">
<!-- http://your.host/assets/css/app.css
NOT FOUND (but still ok when the app base is "/" and won't change) -->

<link rel="stylesheet" href="../../assets/css/app.css">
<!-- http://your.host/larapp/assets/css/app.css
OK BUT this kind of relative url is bad (search the web for more info) and above all you have to take into account the actual route -->

<link rel="stylesheet" href="/larapp/assets/css/app.css">
<!-- http://your.host/larapp/assets/css/app.css
OK BUT if the app base changes to "/gotcha/larapp" at some point, you'll have to rewrite it (e.g. do a find-replace) -->

{{ HTML::style('assets/css/app.css') }}
<!-- <link rel="stylesheet" href="http://your.host/larapp/assets/css/app.css";>
I GO WITH THIS ONE -->

Of course, the advantages of url helpers for actions and routes are way more evident.