Custom inserts, boxes; internal lids where the box lid doesn't stop the bits from moving around; stiffening the top of big game boxes where they've dished. Architects - for example - use it for scale models. You can also mount pictures/posters on it and - if you protect the edges with a wrap-over tape or edging of some kind - player boards. It's a very versatile material.
It cones in 3mm too. Some applications (box stiffening) might be better with the 3mm, but 5mm is the norm for inserts and suchlike.
Do *not* visit "The Range" if you want storage boxes but it's a bit close to pay-day. They have LOADS. Some are perfect for e.g. Caverna. They have (or at least had) 'soft' plastic ones - not the brittle acrylic that cracks easily - with movable internal dividers .
Other folks will be along with more experience than me, but I reckon your blade wasn't sharp enough and thus pulled some particles out rather than slicing through them. Having said that, I'm not sure the composition is uniform all through the board.
I reckon your blade wasn't sharp enough and thus pulled some particles out rather than slicing through them.
That was my immediate thought as well. If your blade isn't super-sharp, then bits of the foam are going to get caught on the blade and pull out of place, leaving holes behind them. The caught foam then often compresses, so you don't see bits of it come out or anything.
The knife will blunt very quickly while cutting foamboard. Best is a razor blade, or a scalpel, but failing that snap off your snap-off utility knife blade after quite literally five or six cuts maximum.
For what it's worth, I've never seen foamboard that isn't uniform all the way through.