Discrediting Travis (read: vendetta) is the means through which Benchmark seeks to protect their investment.
In order to liquidate their investment, Uber needs to IPO or Benchmark needs to find another investor to buy them out.
Travis is a staunch opponent to an IPO. And, replacing Travis as CEO is a likely pre-condition to finding another investor who will buy Benchmark out. Either way, discrediting and removing Travis and protecting their money is more or less one in the same.