It's pretty normal practice. Usually stapled down, it also saves the carpet blackening from air blowing through the gaps in the boards. I don't lift a lot of carpets but pretty much all of the ones I have recently have had this under the underlay.
It seems to be good enough for the majority. Probably beats ply on price/availability. You might want to get a stapler of some type. Depends if you have compressor but there are plenty of manual types.
It's pretty normal practice. Usually stapled down, it also saves the carpet blackening from air blowing through the gaps in the boards. I don't lift a lot of carpets but pretty much all of the ones I have recently have had this under the underlay.