*shrug* I wouldn't have thought it a 'new' use. Trodes are one of the standard ways to get DNI (p217, the only non-implant one), and the primary use of DNI is mental controls. It was indeed a change from SR3, because there used to be no non-implant DNI (IIRC). See also Unwired p42 ("Nevertheless, the ability to simply think your commands is an incredible ease-of-use factor that makes trodes worth the risk.") and p58 ("If your smartgun is wirelessly linked to your smartgun implant, you can control that via DNI as well."). No clue what a 'smartgun implant' is, but the rest of the paragraph makes it clear they're talking about DNI controls, inc. trodes.
DNI is, of course, also required for simsense and VR, which brings us right back to the trodes.

I mean, you could theoretically use a smartgun with contacts (image link) for the targeting only, and use trodes for the control aspect only. It's just more complicated.