Why segment your market?
1. All customers are not created equal
Even when different customers buy similar products (e.g. a monthly public transport pass), you have no way of knowing if they use them in the same or different ways. It is also possible that customers would prefer a different product but choose "second best" because what they really want is not offered.
Without research, you have no way of knowing who your customers are and what they really want. You may in fact find that it is easier and more effective to address marketing activities to a series of smaller and better-definied groups of customers than to use a "one-size-fits-all" approach that is so broad that may not really appeal to anyone.
top photo: user LingHK at Flickr/bottom photo: Adam Jones adamjones.freeservers.com