MAC Address Registration
You're not really missing anything. 

Some organizations release only a handful of bespoke automation devices (often reflashing a generic OEM MAC), while some are full OEMs, launching high-volume products. 

Finally, some organizations are both, and may have budgeting requirements that make it easier to purchase an IAB for a project/product line, than to coordinate internal use of a company-wide OUI. IAB's also allow you to advertise different brand identities for each project/product.