Don't know how familiar you are with the source work but it evoked a simple time in American life of the midwest where kids could roam, have adventures and use their imagination. Less about the actual island to book relationship than appealing to midwesterner's nostalgia. An idea from a previous era of theme park design.I have to admit that I've been to Disney over 30 times; Magic Kingdom probably 20-25 times. I have never taken the Riverboat to Tom Sawyer island. I just never got it.
There were so many of those places built into the original design. Not all were intended for expansion but over time they have been monetized or sanitized for "flow". sad....Tom Sawyer's Island is a great spot to escape the midday crowds and relax/people watch for a bit.
Despite what some of the "ride as many rides as possible" people may think, a well-rounded theme park should include some restful spaces.