A custom computer group is a list of computers grouped together depending on how they need to be managed and secured. Computers that require similar set of security settings could be put together to facilitate collective management. For example, computers used in the sales department of your organization could be grouped together so that Chrome extensions specific to sales could be distributed to the group, instead of one computer at a time.
A custom site group is a list of web-applications grouped together depending on their requirements. Web-applications that are well trusted in the organization can be added to a group and policies with minimal security could be applied to the site-group. Similarly, web-applications that are potentially dangerous could be grouped together and strict security policies could be applied to the group.