On receive, engaging an ATU to lower SWR will not increase signal to noise ratio (SNR) and therefore signal intelligibility on the HF bands. Yes, the signal will appear “louder”, but so will the noise (remember that noise is signal too), and the net result is no change in SNR.
What can make a major difference on receive is the use of directional antennas, which reduce noise from directions other than the desired signals, and therefore may increase the SNR. That’s how specialized “receive” antennas like beverages work. They have negative gain in all directions, but reject noise from undesired directions, and therefore can increase SNR. Using diversity receive can make a big difference too.Howard