As far as I've always believed there is 3 reasons for it.
I. Theory the high temperature shocks the skin helping the skin crisp up if you've ever put a chip in cold oil you'll notice it takes far longer to crisp up than one dropped into the same oil but once hot. Or a better description is probably ... Have you ever put a poppadum into cold oil? Notice how it never puffs up, just goes brown.
The high heat helps seal the meat I'm theory helping keep the moisture in, even though most of the 'moisture' in belly pork is actually the fat self basting itself.
The high temperature helps raise the interior temperature to above the 'danger zone' quicker.
Now I do agree with you, the initial blast does not make the skin crackle, especially since I cover my meat with foil after the first blast which essentially means the skin then steams for 5 hours. I did however try low from the start once and the end result was a thin crispy but a little chewy almost crackling layer even after half an hour at 230c at the end. So I do think high initial heat plays a crucial role to getting good crackling in the long run even if it doesn't seem like it to begin with.
I would suggest not fixating on any time frame for the initial blast. Keep your oven on full wack until the skin bubbles up, if I can be bothered (it's a bit messy and risks major burns every time) I pour 300c oil over the top of my pork half way into my first blast which really helps the skin blister and results in perfect light crispy skin at the end.