Hello Ray
Depending on the platform, the default state of bridge assurance may differ. On the 6500 series for example, bridge assurance is enabled globally by default and is enabled only on spanning tree network ports that are point to point links. On Nexus switches, it is enabled globally but disabled on individual ports and must be manually enabled.
The feature is specifically used to protect against a unidirectional link failure or other software failure and against a device that may continue to forward data traffic when it is no longer running the spanning tree algorithm.
Now there are times when you may want to disable this feature (or enable it on a port where it is disabled by default). Such instances are if you have a device on the other side of the point to point link that may not support it. If it is enabled only on one side of a link, the port will go into the blocking state.
You can find out more about how it works (at least for the 6500 catalyst platform) at this Cisco Documentation.
I hope this has been helpful!
Laz