“Bench strength” here is a sports analogy. On a sports team, the team has starting players but it also has back-up players who sit on the bench when the starters are playing. If the back-up players are good, the team has “bench strength” because the team can send in good players when the starting players get tired or injured. By analogy, in business, a company has “bench strength” if the whole team (and not just a few people) have strong skills. Sometimes, as in the sentence you cite, a business may want to add a person with a specific skill set to add to the team’s overall strength.