The shaking separates the followers of Christ from the people that merely "attend church".
Check out this YouTube video about the shaking by Ellen White.
Also check out this YouTube video about the shaking by Doug Batchelor.
This is the order of events:
- The straight testimony
- The shaking
- The latter rain
- The loud cry
The shaking is not just an event, it has been going on for years (but will be more severe as time goes by).
God wants to have a group of people that are committed to Him, and are able to give the final warning to this world.