I’ve recently had some boost spacing questions so I thought I’d comment here on it.
These are DT Swiss 350 12mm hubs in 135mm/142mm and 148mm(Boost). Notice the disc rotor is in the same place (.3mm) but the right side spoke flange is farther outboard (+3mm). As mountain bikes have evolved, manufacturers have added gears to the right side of the rear hub, and a rotor to the left side, and then still more gears on the right. All this was added to the same 135mm rear wheel spacing by moving the spoke flanges farther inboard.
The profile of a wheel is a triangle. Two sides are made up of the left and right side spokes. The third side, or base of the triangle, is the distance between the spoke flanges on the hub. Given the evolutionary parts changes the base of the triangle has gotten smaller and the wheel less stable. Also, wheel sizes went from 26” to 29er, making the triangle taller and even less stable.
Hub spacing did change from 135mm to 142mm before boost, but manufacturers chose to use the the same hubshell with wider spacers. This made the hub wider but didn’t change the distance between the flanges. With boost spacing manufactures are actually moving spoke flanges farther apart. This makes the base of the triangle wider and the wheel more stable.