JSP might be old, not fancy, or trendy anymore, but many legacy systems still use it, and there are development teams tasked with maintaining and extending systems with a JSP frontend (see https://webtechsurvey.com/technology/javaserver-pages). What can you do when you need to work on a code base that has unit tests for the Java code, but a significant part of the code base is living in (an untested) frontend code and is prone to failures?
You can rely on code reviews or pull requests, but that seems insufficient to flag even trivial issues. You can wait for manual testers or automated UI tests to find problems after the change was deployed to the QA environment, but that is way too late and cumbersome.