From R. Kent Dybvig

​Reference

One value of opacity is that it simplifies program analysis, both by the user and by the compiler. When programming, it's nice to be able to scan the code where a record type declaration is visible and know whether a field is used, whether it is assigned, and what type of value it holds. If any part of the program to which we pass the object can obtain a field accessor or mutator, we've lost this ability.