When I was younger my mum would make me wear a helmet. Once I got around the block from my house I would take it off and put it in my bag or on the bars.

These days I almost always wear a helmet. However on some longer rides in the summer it can be really nice to take off the helmet for a little more ventilation and wind in your hair action. The bars are the most conventient place to put it without needing to pull over. I don't think thats what the OPis really talking about though. I never ride anywhere in the city without my lid firmly attached to my head.