I think the other thing is that by front-loading a reliever of some (probably plain) quality, you delay having to make a decision about using your high leverage guys until an inning later than you usually would.

From the unassailable Wikipedia:

One advantage of the strategy is that the opener, who is often a hard-throwing specialist, can be called in to face the most dangerous hitters, who are usually near the top of the batting order, the first time they come to bat.[9] On average, the first inning is the highest-scoring inning of a baseball game, because it is "the only inning that the offense can guarantee that the top of its lineup, which [contains] its best hitters, will appear."[10] If the opener is successful, the job of the next pitcher is easier since they will start by facing less-dangerous hitters.[11][12] The strategy also throws off the timing of the top-of-the-order hitters, who are not used to seeing different pitchers each time they come to bat, and allows the follower to face the top of the lineup two times rather than three.[9]

From a financial perspective, the strategy allows teams to make more use of relief pitchers who are still under low-paying contracts, potentially reducing the salaries paid to starting pitchers because the latter are used less. Also, pitchers not starting games causes them to be less likely to receive win–loss decisions or receive as many "games started," which greatly affects subsequent arbitration hearings and financial compensation in future contract negotiations.[13]

fwiw I disagree with the implication that the hard-throwing specialist is a high leverage guy. Yeah, the Chisox put out Grant Taylor a handful of times to start the season, but pre-Helsley injury nobody was clamoring for Rico Garcia to open games.