Blocks are never 100% full. If it was then you would never be able to route the design. High utilization may be 70% but if a block has tons of IO then I've worked on blocks that are only 25% utilized. For various manufacturing and yield purposes the empty spaces need filler cells.
Sometimes we put in decoupling cap cells. But the ecofiller cells go in everywhere else.
About 25 years ago we were using spare gates that we had preplaced on the die.
About 5 years ago we started using spare gates preplaced and ALSO the ecofiller cells. The reason I was told was to save money because the ecofiller cells require some other mask layer to change. I think that was in the $500K range but it's still money.
In general I hate doing ECO's with the preplaced spare gates as it is manual and time consuming to find the best cells to use.
> Normally you tell the fab to keep a few wafers after the base layers and don't manufacture the metal layers.
Oh, I had no idea that was a thing.