From the specification : One of the goals of tag files as a technology is to make it as easy to write a tag handler as it is to write a JSP. Traditionally, writing tag handlers has been a tedious task, with a lot of time spent compiling and packaging the tag handlers and writing a TLD to provide information to tools and page authors about the custom actions. The rules for packaging tag files are designed to make it very simple and fast to write simple tags, while still providing as much power and flexibility as classic tag handlers have.
Tag files placed in the /WEB-INF/tags/ directory of the web application, or a subdirectory, are made easily accessible to JSPs without the need to explicitly write a Tag Library Descriptor. This makes it convenient for page authors to quickly abstract reusable JSP code by simply creating a new file and placing the code inside of it.
Note that this is not true for tag files packaged in a JAR file.