To be clear, someone doesn't need to actively search your browsing history.

It's especially to prevent it from showing up in autocomplete. Imagine you tell your mother to look up a plane ticket on Priceline, she types "p" into the address bar, and all of a sudden 8 different porn videos show up as the top 8 autocomplete suggestions.

Now if she's polite she probably won't say anything and you'll never know. But...