A RAM shows a a resource assigned to a work package. This would be used with creation of your WBS, particularly when the WBS is complex and you have Team Members assigned to the project. This can make it clear that one person is assigned to a specific task.
RACI chart is really a form of a RAM. It can be used in the same way as above, but is very useful for multiple people or groups of people assigned to a task. This is also helpful with designation of roles for people external to the project, but are involved. I have been involved with this type of chart many times in my work, due to the large number of folks or groups I work with or the groups I may impact.