From the word’s history
The term exists because nominal value and market value of currency can differ.
錢幣的面額和市場價值有時不同，所以需要這個說法。
Think of a coin lying on the table with a number printed on its front. You read the number and you stop there: that is what the coin says about itself. What someone would actually give you for it is a different question, and the whole word exists because those two numbers can drift apart. Take that habit of reading only the front and turn it on a person's words, and you have the other meaning.
可以想成一枚硬幣正面朝上躺在桌上，上面印著一個數字。你把那個數字讀出來就停住：那是它自己說的值。別人真正願意出多少，是另一個問題——這個詞會存在，正是因為這兩個數字常常對不上。把「只看正面」這個動作用在別人的話上，另一個意思就出來了。