A record is marked as a broken process when it failed to do the respective task. 700k records is a pretty large number. I am assuming these records are there in the system may be from few years.
Now every record indicates the sla for a particular assignment. It is broken when it tries to do the sla and fails to do so which might happen due to lock failure, missing assignment, finishing assignment etc. Now if you are sure, you do not need these SLA actions on these tasks, they can be deleted.
These records are useful, if you want to retry the SLA activity on that particular task.Without this record, you cannot do the SLA action easily. So make a wise decission !!!
If I am you, I would see the age of the records and experiment with few of the latest records to see the failure reason.