I once went for a recognition ride on a race track for a competition just a day before the actual event. The track was exactly the same, it was already marked and it did not cross any fence or busy street (it was a pure mountain biking race). Of course, I went to the competition the next day.
The biggest difference was purely psychological, but big enough to make it for the inscription fee. Most the reasons have already been mentioned, but I want to stress out that the fact that you are sharing the way with a lot of other riders, you can learn a lot, watch different equipment setups, see how other people have advantage over you or vice-versa. You can also get the chance to "teach" or couch somebody else and hopefully you'll meet new friends/ride partners. It is amazing how much practical knowledge is shared among cyclists during these events.
There are a whole lot of other psychological aspects linked to the advantages already brought to the topic (logistics, mechanical aids, drafting, etc) like the possibility of impressing a few couch potato friends by showing you actually accomplished a huge challenge, or simply being cheered up by a lot of strangers near the end of the track. I can guarantee you'll get a rush of energy that will make you forget how tired you are and pedal with all your energy trough the end (even if it's not a competition!)
At the end of the course you'll get involved in all kinds of conversations that will make the event last longer and you'll remember for the rest of your life.
In my case, I usually ride a lot in paths later used for races or "mass cycling events", and I can surely opt in for the paid ride almost every chance I get, the experience simply has no match.
Another advantage (at least for me) is the chance of getting action pictures taken of you while you are completely focused in riding, that look a lot better than when you try to get them by yourself!