Purpose of barrel adjuster on Travel Agent?
It functions much the same as a barrel adjuster on a brake lever or caliper. 


  A barrel adjuster will allow the fine adjustments to the tension of control cables. Screwing it out - anti-clockwise - extends the length of the outer-cable run while leaving the length of the inner-cable unaffected. Thus the inner-cable gets tighter...
  
  Barrel adjusters are used in braking systems to compensate for brake-block wear. The adjusters are usually designed and positioned so the rider can turn them ‘on the fly’. Barrel adjusters on gear-cables allow fine tuning to make index systems work precisely


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It is a nice thing to have but not necessary, it simply helps adjust tension for wear and fine tuning. 

They are helpful in that you can tighten the brake cable slightly after the pads wear or the initial cable stretch without having to adjust it at the caliper. Although they do not replace routine maintenance and caliper should be readjusted after initial cable stretch which is normal with new cables after several rides/miles.