An interposer doesn't have to have any breakout pads or pins. It just needs to sit between a board and component. It can serve as a breakout for components that

An interposer doesn't have to have any breakout pads or pins. It just needs to sit between a board and component. It can serve as a breakout for components that may or may not be compatible with the footprint on the board or may not break anything out at all. I've used one to substitute a much cheaper FPGA in for our very expensive one on the development prototype. We didn't need all the features while sitting on a test bench so we saved ourselves some money until a production unit could be our dev platform. We had a few breakouts on the board of some signals that were difficult to access elsewhere but most just routed right through the interposer. We also developed another interposer that replaced a bunch of deadbug components to prevent rework from occuring due to poorly placed deadbugs. There was enough money to get a little card made but not enough to respin the whole board with the wishlist of other changes we wanted to make.