It needs to be interesting

Do everything what we learn should be useful for us?
I believe so. In present days, we got no time for spend it on useless activities. But do we always know what would be useful for us? Do we know it, when we first time hear about something? All those blogs that we are reading, all those books, is there always something useful? Sometimes answer isn't so obvious, sometimes is. Sometimes even happens that we learned something, but life changes and knowledge which was never used before probably never won't be used.
But do we think that was a waste of time? I can answer only for myself, but many times, when everything changed, I didn't regretted what I've learned, even when I knew that knowledge won't be useful for me anymore.