Chrome's address bar already offers to translate a foreign-language page automatically, so many people never need a separate extension. A dedicated translate extension adds value for translating a highlighted sentence without loading the whole page in another language, or for typing and translating text before pasting it elsewhere.
Chrome's built-in page translation
When you open a page in a language different from your browser's, Chrome shows a translate prompt automatically, or you can right-click the page and choose Translate to. This covers whole-page translation without installing anything, and works well for reading full articles in another language.
Translating selected text instead of a whole page
A translate extension becomes useful when you only want to translate a sentence or paragraph, not the entire page. Highlight the text, and the extension shows a popup translation, which is faster than translating a whole page just to check one unfamiliar phrase or line of text.
Translating text you type
Some translate extensions let you type in your own language and get a translation to paste into a chat, email or form on a foreign-language site. This differs from page translation, since it works in the reverse direction, translating your outgoing text rather than the page you're reading.