Viewed in isolation, it may seem that turning a ride into a DiY Audax is just unnecessary admin. But they enable you to qualify for some significant awards, notably Randonneur Round the Year (RRtY), which for some makes them extremely worthwhile, and well worth the modest admin fee.
Whether they were envisaged as a gateway to Audax I don't know. Certainly they're the reason I joined, something I did my best to make clear in the survey of members earlier this year.
Signing up to ride someone else's idea of a good route on a date which frequently isn't ideal, with no ability to predict the weather far enough in advance and all the awkwardness of getting to and from the start? Most of the time, that's not for me. But back in 2015 when I was trying to rediscover cycling I did do a couple of 100km calendar events, with significant travelling required, and they were very important at the time.
BIB - don't forget the first thing you need to do is buy virtual brevets, and this can be done now. If your organiser is anything like mine you'll get a friendly welcome, some useful advice and an invitation to ask questions.
One thing Audax frequently does is appear to make things complicated. A DiY Audax requires just two things: a predicted route sent to the organiser before the ride starts, and a matching track sent afterwards. The entry form helps to keep things tidy but the system behind it is relatively informal. Suppose, for example, you were getting ready for a ride one morning and found that the Audax site was unavailable. If you simply emailed the route to the organiser with a few words of explanation, that would be sufficient to get the ride validated. Any other details, such as purchase of virtual brevets, can be dealt with later.
The only thing I can think of on the DiY by GPS entry form which might be confusing is the section on controls. Apparently they're needed because the definition of a brevet requires it to have controls spaced periodically. On a DiY by GPS they serve no purpose other than to satisfy the definition of a brevet. Just pick spots which it's easy to pinpoint, such as bridges. They don't need to be where you expect to stop.