CCNA, CCNP, MCSA, CCNA Final Exam, All Answer Test Module With 100/100

This one is bothering me for a while now :) Suppose we have a variable - why is writing a Set function better practice than simply modifying variable's data (and setting the variable to be public instead of private)? It's less coding that way and I can't see any "security" issues.

I want to use the contacts API but for my specific application my users will most likely not want these contacts polluting their contact list. The application is targeted for delivery drivers and I want to use Contacts to keep track of past deliveries. I don't really care if the user can edit or change the contacts, but I don't want to confuse the user by having these contacts start populating in their address book. Any ideas? Otherwise I will use a SQLite Database but I hate re-inventing functionality.