While not eliminating the need for regular piano maintenance, humidity control will allow more stable tunings by reducing the radical pitch changes your piano may experience through the seasons. When your piano stays closer to its correct pitch level of A440 (440 cycles per second), your technician does not have to perform a large pitch raising or lowering procedure prior to fine-tuning. Thus, a balance of forces is maintained between the strings and the frame of the piano, allowing more accurate and stable tunings to be done.
In addition, a stable environment will help to preserve your piano through the years. Wood parts, glue joints, metal parts, and your piano's finish will all last longer if not subjected to excessive humidity swings. Maintaining the correct environment will preserve your piano investment for a lifetime of enjoyment.