After a region is loaded with new data and RUNSTATS is run , does BIND have to run or can just REBIND be run if there hasnt been any SQL changes since the last load and Runstats?

I think REBIND should suffice.

When would BIND be mandatory?

You need to explain what DB2 product and release you are using. What are thinking about binding (plans, packages) and what are they related to (applications, stored procedures, DB2 utilities, etc)?

A bind will create a new package or plan (depending on which DB2 is used). Rebind will recreate the access path for an existing package or plan.