Why do biologists give organisms scientific names?

There are a couple of reasons.

1. It sounds cool.
2. You sound intelligent when you use them.
3. More seriously, it helps distinguish exactly what organism you're talking about. "Firefly", for example... there are over 2000 different species of them, so the scientific name is a lot more, well, specific about which species you mean.
4. It helps group different species into relationships.

A little more information on that last one might be good. If you know that Panthera leo is a lion, you can guess that Panthera pardus is another Great Cat (the leopard, as it turns out). Also, it tells you that while certain domestic cats (Felis sylvestris domesticus) may look more like black panthers (Panthera onca, which is actually a good example of 3 as well, since it tells you I mean Latin American black panthers, which are a type of jaguar; "black panthers" in Africa or Asia are leopards) than lions do, lions are actually more closely related to panthers since they share a common genus.