with regular <script> tags, you cannot control their loading and executing behavior reliably cross-browser. Some new browsers will load them in parallel but execute them serially, delaying execution of a smaller (quicker loading) script in the pessimistic assumption of dependency on previous scripts. Older browsers will load and execute them one-at-a-time, completely losing any parallel loading speed optimizations and slowing the whole process drastically.
All browsers will, however, block other page resources (like stylesheets, images, etc) while these scripts are loading, which causes the rest of the page's content loading to appear much more sluggish to the user.
LABjs by contrast will load ALL the scripts in parallel, and will execute them as soon as possible, unless you express an execution order dependency in the chain by inserting .wait(). In addition, you can "couple" inline script logic to execute in the proper order in your chain as desired by passing a function to .wait(...).
It's important to realize that explicitly, separate $LAB chains operate completely independently, meaning there will be no explicit waiting for execution order between them.