No matter how cheap and convenient the Internet is, many mom-and-pop businesses, most of whom are immigrant businesses with limited English abilities, are simply NOT going to advertise online or have a web site at all.
And what if you're in a hotel with no internet access? The physical Yellow Pages, as antiquated as it is, is still a useful source of information on certain local businesses such as taxi companies, laundromats, barber shops, internet cafes (ironically), etc.
Even for big chain businesses, their web site often doesn't tell you what their hours are for the location you're interested in. So, sometimes I use the phone book to get their number, call them (gasp!), and find out their hours.
And yeah, like someone said before, many older folks simply aren't going to use a computer, and will use the physical yellow pages.