The manager is better informed and more up-to-date with his or her people's activities (and more in touch with what lies beyond, e.g., customers, suppliers, competitors, markets, etc)
Difficult issues can be identified, discussed and resolved quickly, before they become more serious.
Help can be given more readily - people rarely ask unless they see a good opportunity to do so - the regular informal review provides just this.
Assignments, tasks and objectives can be agreed completed and reviewed quickly - leaving actions more than a few weeks reduces completion rates significantly for all but the most senior and experienced people.
Objectives, direction, and purpose is more up-to-date - modern organizations demand more flexibility than a single annual review allows - priorities often change through the year, so people need to be re-directed and re-focused.