I think this sums it up pretty well. For a while one can progress on almost any kind of haphazard program. And truth be told, that is good enough for most recreational riders, even those doing group rides and dabbling with racing. How many people's livelihood depend on increasing their speed by .5 mph or slightly increasing their FTP or VO2 Max? But when you get to the point when doing this is necessary or a goal you feel is worthwhile for some other reason, structured, science based training is necessary.