"Transformations" are essentially automations for all the little annoying, recurring tasks you typically have to take after moving a document into your website.
For example, take "open links in a new tab."
Typically, you'd have to manually:
Copy and paste the target="_blank" attribute to each individual link, or
You'd have to click on each link inside your CMS and enable the "open in new tab" option (assuming they actually have it built in, which many don't).
Editing a single link might only take a few seconds. But multiply that by a lot of links in each document, on top of other things we automate like image compression. And then think about how many times you're publishing over and over and over.