Relation between reflection (coefficient) and bandwidth
As your signal propagates through the transmission line, when it hits a discontinuity, it will reflect and start heading back. If you have multiple branches in your transmission line, each time you come to one of those intersection, you'll have part of the signal travelling in each of those branches and then back (if the branches are not terminated properly).

A signal travels at a finite speed, and if you have a fast frequency signal, it may change faster than the signal can propagate through the line. So now you have your signal interfering with its past self (reflections) which will then distort your signal. 

By minimizing your reflections, you minimize the amount of interaction between the reflected signal and the incident. This then decreases your ISI (inter symbol interference), which allows for more data to pass through without issues and therefore a higher bandwidth.