This is taken from the documentation:
Hi - I guess by now you understand the basic definitions of these different Virtual Disk Provisioning Policies through articles like vSphere Documentation Center
However, would suggest you to check this Eager thick vs Lazy thick disk performance | Rickard Nobel to understand these disk provisioning policies in detail that would probably help you more.
On what occasions do you use one rather than the other?
you would provision eager zero'd when you need fast write performance. perhaps for a DB or an Exchange DAG. personally I have not used them all that much as the time laying them out is quite long and it is very intensive on your Shared storage.