Keeping track of the buisness hours of a specific office is way easier than keeping track of their timezone, business hours and then convert your local time to their time to see if they're actually open.
You can. But especially if you don't have frequent contact with other timezones it is very hard to keep track about how far they are ahead or behind your local time. This gets even harder with DST.
While this might be easy if you're just in different US timezones, but it gets increasingly harder with larger distances.
