Maybe you have to work on some existing XSLT or XQuery code, whether you wrote it years (or minutes) ago or whether someone else wrote it. Maybe you have to write new transformations or queries.

You have some exploring and some planning to do.

You need to explore the possible input. If you are lucky, maybe there is actual input for you to explore and, later, use for testing. Maybe there is pre-existing code to explore; after you have started work, soon there will be new code to explore!

All of these things can be done with or without tools beyond a simple text editor, an XML well-formedness checker, an XSLT or XQuery processor, and so on. But if the input is marked up, we can explore the markup, and maybe tools, whether off-the-shelf or custom, can make that much easier.

A strength of XML and especially XSLT is enabling people who do not think of themselves as programmers to do advanced text processing. It follows that you do not need to be an experienced programmer to write useful tools.