Almost every word in the English language has a synonym. Measurements like "mile" and "ounce" don't have synonyms. Science terms like "planet" and "photosynthesis" should stand alone. Words like "unique" and "parallel" are pretty specific, and ironically, the words "synonym" and "antonym" don't have any synonyms, but other than that, there's a synonym for everything.
A lot of synonyms are somewhat of a stretch, but if we had a bunch of words that meant the same exact thing, we wouldn't need them. The words "avoid" and "evade" technically mean slightly different things, but they're close enough they can be interchanged. Unless you're preparing legal documents, synonyms are a fun way to read and write.
The more synonyms you know, the better reader and writer you are. If you simply read the same words over and over, you'd eventually stop reading. It'd get too boring, and your writing would be even worse. Every now and then, you need to include a "brouhaha," "cacophony," "fantoosh," "lackadaisical" or "vomitorium" into your vocabulary. They're all complex words with simple meanings, and anyone who has read anything knows what they mean.
How well do you know your synonyms? See if you can correctly replace the words in these questions with the proper synonym.