Teams like saving money and extending their control over top young players. Why wouldn't they? Having impact players on affordable contracts simplifies a GM's job. As a result, teams call top young players up strategically every season to control their service time and, in doing so, delay their free agency and/or limit their earnings.
Though service time is a consideration all season long, it's most evident at two times: in April and again midseason, around early June. If teams wait until a few weeks after the season has begun to call a prospect up for his MLB debut, the player doesn't collect a full year of service time, which delays his free agency by a year.
The precise date until which teams must wait before calling prospects up varies each year and according to whether players are on the 40-man roster. Now that we're nearly three weeks into the season, even prospects on the 40-man roster can be called up, since they have spent the requisite 20-day period in the minor leagues.
None of the following prospects have big league service time, which means that their teams can call them up at any point and keep them through the 2017 season, if not longer: Dustin Ackley, Lonnie Chisenhall, Brett Lawrie, Mike Moustakas, Jesus Montero, Eric Hosmer, Julio Teheran, Manny Banuelos, Kyle Gibson and Jordan Lyles.
On the other hand, Michael Pineda, Zach Britton and Brandon Belt are now in the majors, picking up service time. Because those players are now on MLB rosters, they're currently on track to hit free agency after the 2016 season. However, if their respective teams option them to the minors for 20 days or more, their path to free agency could be slowed as well (that's an immediate possibility for Belt and a long-term one for the pair of impressive rookie hurlers).
That may sounds complicated, but it's the easy part. Later this spring, in late May and early June, the guessing game begins. Teams do not (and can not) know exactly when future cutoffs for super two status will be, so if they want to play it safe and ensure that prospects like Montero and Ackley only go to arbitration three times, they'll want to wait until at least the middle of June before calling them up.