Organized tours are not for everyone, but I think they are a lot of fun. Some reasons:
-- Routes designed to minimize traffic and maximize scenery and attractions, with cue sheets and maps provided.
-- Organizers carry your luggage so you can just ride your bike rather than lug around a bunch of gear at a slow pace.
-- Free camping (as part of entry free) and often transportation to nearby hotels for an extra charge.
-- Opportunity to ride with other like-minded people, make friends, socialize.
-- Costs often very reasonable if you camp out.
-- SAG support as well as rest stops with food, drinks, mechanical support.

The main drawback to organized tours is the sheer numbers of other cyclists. Some of these rides, such as RAGBRAI in Iowa, can attract thousands of participants. That can make riding sketchy when caught in a pack of inexperienced or unsafe riders, and campgrounds crowded and noisy. Food stops can be crowded, with waiting lines. For those reasons, I personally avoid large rides like RAGBRAI that attract thousands of riders. However, there are plenty of other organized tours that limit the number of participants or aren't as well known, such as Cycle NC, Bike Virginia and TRIRI.