Why do people change positions within the same company?

People change position within the same company for a few reasons:

1. Promotion - they do their job really well and have the skills to take on a role with more responsibility.
2. They want a promotion but it isn't always possible within the same department so they change positions to something they are qualified for as well.
3. They do a poor job in their current role and then a better position is matched for them.