This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
Back in the Ice Ages, when it was appropriate to include scriptlets in a JSP, this was intended to identify the scripting language, just in case any other language was ever supported in scripted pages.
Luckily, before that could happen, scriptlets went out of vogue and have been replaced by the JSTL and EL.
So the directive is doubly useless nowadays because:
No language but Java ever ended up being supported.