First, here is a press release from the New York Department of Environmental Conservation on a recent progress to stop the illegal trading of native reptiles and amphibians from New York state and nearby areas.
So why is the illegal trade of wild animals such a big concern? Not surprisingly, there are lots of reasons.
Lets begin with the most obvious reason: to ensure the health and persistence of existing wild populations. The demand for many wildlife species for food, pets, and/or for traditional "medicines" has lead to the drastic reduction in wild populations worldwide. There are countless examples of this happening all over the world. For example take the story of the Spix's Macaw (Wikipedia | birdlife.org) which went extinct in the wild in 2000.
For other examples, you can read more here from NPR and here at america.gov. To make a long story short - we humans do a pretty good job of wiping out wildlife unless we practice some restraint!
So what's our secret to being such efficient exterminators? Going back to the recent arrests in New York state, it's worth mentioning that these collectors endanger wild populations on a number of levels. The obvious harm comes from the removal of individuals from sensitive populations, which can lead to significant and long-lasting population declines. This is what we usually think of as the main problem. Additionally, however, the destruction of local habitat can also have a lasting negative impact on wild populations: see the results of this study for an example. Unfortunately, this fits a general pattern: overharvesting wild populations while at the same time reducing high quality habitat seems to be a great recipe for decimating a species.
Some people, perhaps you yourself, don't find this all that big of a problem - "so a few species go extinct - big deal!" Arguments against this sentiment can be found here and here, so I'll leave that issue alone for now. But even from this very extreme perspective, there is still more to be concerned about beyond species conservation alone!
Other big reasons to keep tabs on the trade of wildlife (reasons often overlooked in the public eye) are concerns about spreading disease and invasive species.
Wildlife can carry diseases that can be harmful to humans, livestock, and/or to the wildlife in other regions. Illegal trade in wildlife has a huge potential for the spread of costly diseases at a global scale - for example consider the recent worry about the global spread of bird flu, and the fact that SARS originated in bats or other wildlife before infecting humans. For more on wildlife and disease concerns, see this article and some of the other information from the U.S. Centers for Disease Control and Prevention (CDC).
Invasive species can also cause a great deal of environmental, agricultural and economic damage, for examples see the impact of invasive species and check out the relatively new (2005) government website for the NISIC. For more on the invasive species side of the illegal trade in wildlife, check out the (more comical) hippos in Columbia, or the information available here.