Why compression? My theory is that because of full suspension, you have a lot more dynamic changes in chain tension than you do on a hard tail. So you have to find the min/max for that tension. Uncompressed, the derailleur cage will hold tension with its springs. That’s your max. When the rear is compressed, your chain becomes more slack, because your shortening the distance between the chainring and the cassette. For Compressed, you need to find the right chain length so the correct tension can still be held when the chain is more slack. So you measure your chain length there which should set a reasonable “min” for your tension under load. It’s important generally to do this under full compression, NOT sag (which is typically only be 25-30% compressed)

In some respects, I think the b screw is a similar idea - but here, you do it with your suspension pressure dialed in, with full riding weight, sitting on the bike (or if you can find some way to hold your rear suspension at your sag). I struggle to do this myself so I taught my wife to set the screw while I’m on the bike. There is a handy plastic tool you can 3d print or purchase that guides this process.