Why are SQL fieldnames sometimes spelled like `this`?

What's the difference between

SELECT * FROM `this`

and

SELECT * FROM this

?

The former is escaped, the latter isn't. Consider:

SELECT * FROM `FROM`

On systems where the the backtick is an escape, that would select from a table called FROM (whereas without the backticks, it's a syntax error). Some systems use square brackets instead, e.g., SELECT * FROM [FROM].