How many forest or domains in your environment and why?

I have been studying for 70-640 (Configuring Active Directory for server 2008) and a lot of it talks about having child domains, multiple domains in a forest, or multiple forest. I have never been in a situation like that working for a MSP, all are single forest/domain with the same name. Our company has our domain and a test domain (lab) but they are physically separated hardware and network.

I am just wondering how many people are working in a set up with multiple forest/domains and why?