HTML Entities — Escape and Unescape Markup Safely
HTML entities represent characters that would otherwise be interpreted as markup, or that are hard to type. Converting in both directions shows you what a browser will actually render from a piece of source, which is a different question from what the source looks like — and the gap between those two is where a fair amount of display weirdness lives.
When it helps
Usually to explain something visible. A page showing a literal ampersand-a-m-p-semicolon on screen has been escaped twice, typically because a template escaped a value that arrived already escaped. A page showing raw tags has been escaped where it should not have been. Converting the source by hand tells you which of the two you are looking at in a few seconds, and that determines which layer to fix.
Worth running automatically
The escaping question matters most where it becomes a security question, and there the useful automation is not conversion but monitoring the result: content-policy headers that constrain what injected markup can do, and scans that check what a page actually renders. Escaping is a code-level concern that no external tool can verify directly, so the practical continuous check is on the defences around it rather than on the escaping itself.
What you get out of it
Clarity about what the browser will render versus what your source says, which is the fastest way to work out which layer applied one escape too many or one too few.