All excavation is destructive, whether done by the best professional archaeological team or by a looter. The archaeologists hope to gain a lot of information through that destruction of the site, but the site is still gone when they are finished excavation. Therefore, archaeologists will often not excavate a site unless it is going to be destroyed (by construction, for example). They may prefer to leave a site, or at least a part of the site, intact for future archaeologists who may have better techniques to get even more information from the site. Remember, archaeologists collect information, not just stuff.
But many times, sites are going to be destroyed by construction, and excavation is the only way to recover the information. Then, the archaeologists try to recover as much information as possible, bringing back both the artifacts and documenting their context, where the artifacts were, what they were associated with, and so forth.
Conscientious amateur archaeologists who conduct surface collections, document their finds, and report them to archaeologists serve a valuable role in archaeology. Many archaeological sites have been reported by farmers who have found artifacts on their property, or by amateur archaeologists who have walked over many fields looking for sites. When these people report their finds to the local archaeologist, the information becomes part of the bigger picture and we get a better idea of what happened in the past.
However, amateur archaeologists can also endanger archaeological resources when they dig into a site. Because all excavation is destructive, digging without the proper training and supervision provided by a professional archaeologist leads to the loss of valuable information about the context of the artifacts, and can destroy a site. There are many opportunities for amateurs to get involved in appropriate excavations, including the Passport in Time project run by the Forest Service. Organizations such as the Mississippi Valley Archaeology Center run public field schools where the public can learn about the process of archaeology through engaging in ongoing projects. The Archaeological Institute of America and Earthwatch also have many exciting opportunities around the world.