I read the syntax and use on MDN, a book, and other places but cannot realize the practical value of the tag. Search engines perhaps?
A common comprehension issue individuals face when starting to deal with semantics markup is to forget that it explicitely brings no value to the style or functionnality of the web content you produce.
Semantics helps other products or purposes that an HTML author is usually not involved with directly. The underlying intent is to come up with a Semantic Web that would be understood by a large variety of artificial intelligences, accessability systems, lurkers, etc.
Although the following isn't an example that necessarly requires the <dfn> tag; someone asking Google/Siri/Cortana "what does ONU stand for" will be glad you used the appropriate semantic markup to get a valuable answer.
The dfn element represents the defining instance of a term.
<p>The <dfn><abbr title="Garage Door Opener">GDO</abbr></dfn> is a device that allows
off-world teams to open the iris.</p>
TL;DR: Semantics are practicals, but usually help other systems more than yours.