From Sonic Retro
Robot transporters appear in every first and second Act of Sonic CD - if Sonic travels to the past and destroys them while they are still active, the act will gain a Good Future. Robot transporters exist in other time zones, but are inactive, merely giving a clue for where they were placed.
Sega did not offically name this object in 1993, and while clues to its existence exist in the Japanese manual, no references are made at all to it in documentation produced for overseas markets. It is only casually named as a "robot transporter" in the 2011 remake of the game, though even then the game refers to it as a robot teleporter in one of the achievements.
While theoretically the robot transporters give the player an incentive to use time travel, their usefulness is mitigated by the existence of the Time Stones. If Sonic completes all of the Special Stages, every Zone will be given a good future, meaning the good ending can be accessed without needing to interact with a single robot transporter, or indeed having to use a Time Warp Plate at all. Likewise, however, the good ending can be achieved by destroying robot transporters and avoiding Time Stones, or a combination of both.