I don’t understand why this ‘function’ exists in a mastering program. If, for example, you are routing through EXT EFX there’s bound to be noise entering the finished product. It’s unavoidable. This noise might be minimal, but it’s still there, and with a good analyzer you can see it. (However, in WL, it takes some effort). Also, with something like RX9, you can remove a fair amount of it if you place it after the EXT EFX slot. And here I’m talking about typically 60 cycle derivatives, but there might be some hiss, as well, depending on your routing and what external devices you are using.
In WL the only way to check this is to put an analyzer on the last plug-in slot (or at least after your external efx device or devices). From here, assuming your analyzer can drop down to -117db, you have to play your file and wait till it gets to almost the very end (when the wave file becomes a straight line). I use a frequency analyzer plug-in from SIR Audio. In my case, I right scroll to the frequency band control button and drag it to the bottom of the window (-117db) and keep my pointer on the analyzer control button so it doesn’t drift upwards.
As a comparison, in Cubase, this doesn’t happen (this muting that occurs in WL) and it’s actually an easy path to see the ‘noise’ in the analyzer. You can stop Cubase anywhere on the playback range of an audio file, scroll down to -117db in the analyzer and here the ‘noise’ of your EXT EFX device’s (or even interface) is plain to see. From here you can make corrections with a denoiser or dehum plug-in or whatever you might want to try.
To close, I suppose you could create a wave file of silence and, provided your EXT EFX choices were static, you could play the file and use your analyzer and make your corrections ‘permanent’ on the plug-in. Sure, but why can’t you just stop play and go from there with whatever external devices you might be using? I think this process, muting the audio, ignores/hides a problem that could be corrupting (or at least altering) your finished product, and I think this is something a mastering program shouldn’t do.