Synonyms are words that have similar, but not necessarily identical meanings.
Here’s an example: Recently I saw a headline that mentioned the auction price of a car:
Ferrari GTO Sold For $48.4 Million, Breaks Auction Record For Most Valuable Car
This wasn’t the only similar headline. My point is that the headlines used the word “valuable.” The correct word is “expensive.”
Value is subjective; it varies from person to person. It tends to have a certain emotional punch, too, certainly more than a mere number, which is what most expensive refers to. (To be fair, I should add that the fact-laden article used the less-emotional phrase “highest-priced car.”)
Emotionally charged words make better headlines, but if you’re explaining something, stay away from emotion; stick with the facts.