The barrel adjuster does allow you to microadjust the tension on the cable. However it does not act directly on the cable. The barrel adjuster effectively shortens and lengthens the cable housing between its two fixed points. By changing the cable housing length you are changing the distance the cable runs and the tension on the cable. That tension is translated into repositioning the derailleur. The shifter and brake barrel adjusters work the same. All adjuster I have seen will work against the housing and not the cable.

Correct me if I am wrong.