Well you're not alone in asking that question. Nine out of ten people I asked six months ago had no idea what a phone card was. The phone time is inexpensive and completely private. After the time is used you can retain the card for its collector value. Unlike baseball cards which have only an intrinsic value, phone cards have a definite monetary value.
Some cards have very limited mintages, while others are produced in massive quantities.
And the variety of topics pictured on the cards is limitless. Cards have been issued to commemorate events, honor famous people, portray famous landmarks and picturesque scenery, artwork, sports heroes, etc. There are themes and topics interesting to all types of people. In Japan they estimate that over 25,000 different phone cards have been issued. The U.S. will be the last major country to adopt them, and it is already happening at a break neck pace.