I will have the following run of cabinets (which will be screwed together and to the back wall) and will install a 20mm (3/4") thick granite countertop over it (with sink and cooktop range).
I've seem people saying that a plywood backer is important, and some other people saying that it is more trouble than it is worth and the cabinets provide enought support as it its. The maximum space between to cabinet walls is about 90cm (3 ft).
Best Answer
The answer to this seems pretty clear when you consider one thing: It takes a substantial amount of force to bend granite, but it only needs to move a few micrometers* to break.
Now you just ask yourself whether a layer of plywood will prevent that miniscule movement. No, it won't. Physics has no interest in the opinion of the masses.
So, you might use a plywood substrate to help replicate a thicker top, or to reinforce poorly-built cabinets, or to help maintain position by adhering the stone to it, or for other practical reasons, but you don't do it to support, stiffen, or strengthen the stone. That would take something much more substantial and rigid, such as the vertical hardwood cabinet rails that typically exist at the front and rear of a cabinet.
* Or a few dozen, or a few hundred. The result is the same.