Why is it that people join groups, just to be members and not active participants?

Question

Why is it that people join groups, just to be members and not active participants?

Answer 1

Many people don't have time or are only on line for short periods of time so their contributions may be limited however sharing information is what  helps many members.

Answer 2

That would be me you're talking about.  

Some groups you have to join in order to read. Others, my reasoning is that I was interested at the time and I will be interested in again later, but I really don't have the time or have anything to contribute. I guess I'm there mainly to learn.