If you want to control your Gain Output to an Amplifier or Active Speakers, etc then you'd have to get TRS to RCA Adapter plugs. This would give you control of the Gain heading out, via the "Main Out" jacks to the Amplifier (Highlighted in White in below picture).
If you wanted Virtual Dub / VirtualVCR, etc to capture the audio from a VHS transfer then just route the RCA leads from the VCR to Input 1 & 2 which you can control via the Gain 1/2 rotational knobs on the front of the unit so you'd manage to control what was going into the PC (Via USB) but not what's running through to the RCA Outs (Unless you install their bloaty software). Gain control is a must with music as if the recording coming in is "Hot" and you have no Gain control, you'll trip the amplifier circuits or induce clipping (bad).
If you wanted to attach, say, a PA / cassette / DAT recorder / Amplifier to the RCA Jacks (Highlighted in Yellow in below picture) then you'll have no control to anything connected via the RCA out's so you would have to either use the Behringer Software on the PC or have a mixer controlling the signal coming in via Input 1 or 2.
I can only assume that the RCA Out's are simply a passthough from the input on the hardware's internals to cut down on production costs? While not a dealbreaker, Gain control on RCA Out's is pretty much standard and essential if you use the card for routing a CD / DVD Player / Turntable, etc as not all vinyl / cd's are mastered at equal levels.
While it's just a case of an additional piece of Software running on the PC, I don't like having more than I need running when capping anything so I'd give it a swerve but that's only my opinion of course.