Juniper Policy Enforcer Used Case and Benfits

Hello,

I'm trying to understand when do we need to purchase the policy enforcer. As per my experience, if you want to control the security on firewalls to push policies and logs, you just get the security director and if you want to manage the routing of the firewalls and switches you get the network director.

1- Do we need it to push policies to third party devices only?

2- If we have the policy enforcer, do we need the security director or they have to be together

3- What's the best case to purchase it?