It can be useful when you want to display single letters of some RTL alphabet embedded in the text using LTR alphabet. Take for example this sentence: "The letters א, ב, and ג are at the beginning of the Hebrew alphabet." I wrote א (aleph) first, then ב (bet) and ג (gimel), but browsers swap the first two letters (and display comma to the left of aleph) because of the BiDi algorithm. When you use bdo element: <p>The letters <bdo dir="ltr">א, ב,</bdo> and ג are at the beginning of the Hebrew alphabet.</p>, the sentence will be displayed correctly.

Unfortunately I can't show it here because apparently StackOverflow cuts out bdo, but here's the example on JSFiddle: https://jsfiddle.net/xwdv8fqz/1/