a restricter is used to lower oil pressure going to the turbo, mostly when the oil is cold.
if you have an oil press. gauge, you will notice, that at start up, the pressure is up in like the 50s-60s, but once warm, and at idle, 10-15 psi is normal.
the turbo seals don't like a lot of pressure- they just want the lube.
the restricter, well, restricts, the pressure.
frankly, IMO, if you have a properly sized feed line, say -3an, i don't think they are remotely necessary.