We all have the same amount of time
All cultures have the same number of hours in the day, but each culture views time differently. Did you know that North Americans think that time is almost as important as money? Sometimes we would rather have time for a holiday instead of the money for working those hours. We value time very highly.
Because of this, when you plan to spend a lot of time talking to someone either in person or on the phone, it is considered polite to ask if they have time to talk. This is especially true of someone in authority (such as your boss, your teacher or professor, doctor, etc.) or someone you are dealing with in business.