Since you asked for practical self-defense and no theory (as if those aspects could be separated), I'll try to stick to practicality.
1. IMHO the only new thing forms teach us we didn't already learn in the techniques, is to feel the logic of angle changes. I mean the way to switch from an attacker at 12, to an attacker at 6 or 9.
Apart from that the forms do provide us a way to isolate your movement from that of the attacker, enabling you to perfect your moves. What I do with my students is first learn the techniques, then the form to perfect the moves, and then back to techniques with a partner to make those perfect moves actually work.
Last but not least the forms provide a way to train stamina.
Could there be found other ways to train those aspects? Of course. We could train angle changes with mutiple opponents. We could train stamina in a techniqueline with lots of opponents and we might learn how to perfect our moves with perfectly compliant opponents (and after that they start resisting to train that too).
But first we train forms to learn the angle changes and to develop perfect moves and stamina, en then we add those other means. The other means are not introduced in the first place because they are harder. We need multiple opponents which are not always available and the excersises require a creative mind because you'll have to make 'm up yourself.
2. The extentions. Of course we could figure it all out ourselves. Actually we could stop after yellow belt end go from there. I guess everybody will understand however that only 10 examples of possible defenses will not be enough to give a student a sufficient amount of suggestions to experiment with. Everybody also understands that 10.000 examples will be way too much, because you would have to spend more then one lifetime to memorize all that before you start experimenting and making things up for yourself. In the Parker system the choice was made that 154 techniques and 96 extentions were sufficient. Is that debatable? Of course it is, it is highly personal. But there's a choice there! If you think the amount of examples is too high, go train IKCA, they have only 56 techniques. If you think it is not enough? Go train Tracy's, they have way more. If you think you shouldn't memorize techniques at all, go train JKD, they don't have any.
Any of these systems are valuable to the ones that train it. My choice is for the Parker system, with 154 techs and 96 extentions. As a side note: we made the choice to train, memorize and require all 154 techniques, and train but memorize nor require the extentions. Every now and then I study a few extentions in written version or from video (since AFAIK there's nobody in the Netherlands that knows them all) and I teach that to our students. We train and learn any new possibility that's in there, keep that in mind and body and forget about the extention itself. It is my choice to spend my energy improving my coordination, power and precision, and not memorizing another 96 set examples.
Just my 2 cents.
Marcel de Jong, 4th Black from the Netherlands