These Verizon and AT&T cards work over cellular signals -- completely different from what you'd get in an airport or coffee shop via their wireless (which is 802.11 a/b/g/n). Your Mac - if it has Airport - has built-in support for 802.11 ("wifi").
You'll need a card from Verizon or AT&T if you want data access away from wifi points, i.e., over cellular networks.