Ad numeram

Many people do X. Therefore X is right.

If other people do something, then it is a reasonable thing to do.

This is often used as an excuse. When we are uncertain about something, we turn to other people and assume they know what they are doing.

This is also a method of influence in social groups, where pressure is put on people to conform.

Adoption of common practice happens when people are uncertain what to do. When the right thing is unclear, it is generally safer to copy others.