Knowing what the answer is supposed to be makes it much easier to reverse engineer the workings, and it lets you double check that your workings are correct. Also many of the more advanced graphing calculators (don't know the TI-89 specifically) have build in CAS systems and can do symbolic differentiation and as such help with showing your workings as well.

Plus the fact that these calculators let you store arbitrary data, so you can have your entire textbook stored in memory if you wanted.