"Extremely useful" is a common phrase used to describe something that is extremely helpful or valuable. It is often used in situations where the speaker wants to emphasize the positive quapties of a person, thing or situation.

Example sentences in Engpsh:

This app is extremely useful for keeping track of my finances.
The information you provided was extremely useful in solving the problem.

这个应用非常有用，可以帮助我记录财务信息。

你提供的信息对解决问题非常有帮助。