We do not need a general consent or confirmations from many people for believing in the necessity of an activity. Because the activities are not ideas or opinions.
When we believe in their necessity, we do not investigate them about their reasons. We do this only when we mistrust them.
For believing in them, we need only to see how other people - a few or only one - do them with confidence. Because such confidence equates with an immersion into reality which makes us to believe that their activities are parts of the reality itself.
We start to learn in this way. The child sees the adults doing a lot of activities with confidence, and then imitates them.
We also continually live for doing a lot of activities deemed to be necessary with the same justification.
Even if the age or the experience tells us that what we do is totally useless, we always see many people doing them.