There are so many ways to memory vocabulary – repetition, making sentences using the vocabulary you want to remember, using spaced repetition software, using a memory technique, etc.
I don’t deny that these methods can help you remember vocabulary. But what if you don’t want to use (or can’t use) any of these methods for some reason? Maybe you’re reading a book that is so entertaining, so much so that after you look up a word in the dictionary, you just want to continue reading ASAP. It’s not the time to take out a pen and start making sentences with that word. It’s not the time to take out your smartphone and enter the word into your vocabulary software.