It could be one of a number of things, and it depends on you and what you want out of it. For example: Company, either friends or people met on the event; Catering - some Audaxes provide nosh; Motivation - doing a set event may put aside thoughts of bailing out, or if you are slow may concentrate your mind on avoiding the cut-off; Going a bit faster (if that's what you want) you may tag along with some faster riders and speed up a bit; Guaranteed tried and tested route; A bit of card as a reminder; A sense of satisfaction/bragging rights "I did the so&so" sounds a bit better than "I went for a ride" (or maybe it doesn't).
This pretty much nails it. Other things come on top: Once you have ridden your bike for more a few years, you'll get tired of the obvious routes at your doorstep, and you will fancy some variation. Moreover, you may get into the idea of doing a RRtY (or , for that matter, enter
CC's century a month challenge) Riding 125 miles on a grim, cold November morning is much nicer when you do it in company.
And another thing is really long stuff. I find it ok to ride 12 or 16 hours on my own, but everything longer than 300k is really quite a grind if you're doing it alone. Audax UK calender events, when you share the agony with others, are much more enjoyable.
Plus, if you get into Audaxing, odds are that you wanna ride PBP, and then you have to do the qualifiers.
you'll never fit in round here with THAT attitude! 
