Yes BBM (BlackBerry Messenger) gained ground initially as a secure point to point messaging for BlackBerry. Very similar to SMS though it often was usable even if the BlackBerry email was down. At one time it also bypassed BES servers (though I think that has since changed).
It was also free. Unlimited SMS for each BlackBerry for free (this was back before free SMS alternatives existed).
Now the security is largely gone as I believe RIM has the ability to gather up info from BBM (most via government request).
I think BBM lives on largely out of habit from those dwindling numbers of BlackBerry users who have a large address book. It's also included free on every BlackBerry sold and works Internationally (all things that were cool when BBM first came out but there are competitive products that offer virtually identical features).
The one feature which is neat (and somewhat unique though PingChat. WhatsApp and KiK all have this ability) is you have automatic read receipts as well as the ability to see when the other person is typing.
In short BBM was cool and hip 5 years ago. Today it's just another SMS substitute that comes pre-loaded on BlackBerry devices so people with BB tend to use it to communicate with other BB users...
RIM is trying to promote BBM as a social platform. Their BlackBerry Music is probably the biggest push in that direction.