Tag libraries are used based on your requirement, say when you need more than what EL can do and standard actions. Tag libraries contain custom tags that you can use to suit your needs, and you can even create your own tags (this is a big process and requires you to write Java code that runs when you use your customized tag in JSP) in most cases accessible tags in JSTL would be enough.
, JSP, (java-) jsp, .. script - , , -, java, jsp, Java-, jsp.
'Head First Servlets JSP' .