Tuck Everlasting is about a family who have discovered the secret to immortality. The reader does not discover how the Tuck family have managed to remain immortal until later in the story, in chapter 7. Therefore, when Babbitt ends chapter 2 by explaining that all the Tucks had "looked exactly the same for eighty-seven years," she creates a suspense which endures for the next four or five chapters. The reader will want to continue reading after this point to find out exactly what happened eighty-seven years ago that has enabled the Tucks to defy time and not age.
The author also perhaps ends chapter 2 with the revelation that the Tucks are immortal to give the reader a better understanding of the Tuck family and their peculiar behavior. At the beginning of chapter 2, for example, Mae Tuck's husband complains when he is woken up because he was "having that dream again, the good one where we're all in heaven." The fact that Mae's husband has this dream so often makes a lot more sense once we know that the Tucks are immortal. Mae Tuck's husband has this dream so often because he is fed up with being immortal, and with the prospect of living forever.
When we know that the Tucks have not aged for eighty-seven years, we can also perhaps make more sense of why they take such care to conceal the little spring in the woods. The reader may at this point be able to join the dots, as it were, and work out that this spring is the source of the Tucks' immortality. Indeed we find this out for sure in chapter 7.